I obtained my PhD degree at the University of Groningen, for studying effects of light on human alertness, thermoregulation and sleep. Thereafter, I decided to pursue a postdoctoral fellowship at Stanford University Center for Sleep Sciences and Medicine. Here, I am developing skills to study a combination of basic and translational concepts of human sleep, in which we try to assess the association between sleep patterns and various diseases, as well as the influence of light exposure on human sleep.

Honors & Awards

  • Trainee Innovator Award, Department of Psychiatry and Behavioral Science, Stanford University (2022)
  • Best dissertation of the year in the field of Behavioral and Cognitive neurosciences, University of Groningen (2021)
  • Young Investigators Research Forum Scholarship, American Academy of Sleep Medicine (2021)
  • Merit award winner based on scientific excellence, Society for Research on Biological Rhythms (2020)
  • Best Open Access Publication, University of Groningen (2020)
  • Travel grant, Society for Light treatment and Biological Rhythm (2018)

Boards, Advisory Committees, Professional Organizations

  • Board member, Society for Light Treatment and Biological Rhythms (2018 - Present)
  • Clinical Advisor, Center for Environmental Therapeutics (2021 - Present)
  • Trainee Subcommittee member, Sleep Research Society (2021 - Present)
  • Scientific Advisor, Good Light Group (2022 - Present)

Professional Education

  • Doctor of Science, Rijksuniversiteit Groningen (2019)
  • Master of Science, Rijksuniversiteit Groningen (2014)
  • Master, University of Groningen, Neuroscience (2015)
  • PhD, University of Groningen, the Netherlands, Chronobiology

Stanford Advisors

Current Research and Scholarly Interests

I'm interested in questions regarding perceived experiences and measured markers of those experiences, for example how do perceived sleep quality relate to sleep quality measured, or how does daytime sleepiness relate to sleep quality at night.
Other interests include effects of daytime light exposure on nighttime sleep, circadian clock phase changes by flashes of light, and how stability and variability in daily rhythms can predict health and disease

All Publications

  • The Impact of Missing Data and Imputation Methods on the Analysis of 24-Hour Activity Patterns. Clocks & sleep Weed, L., Lok, R., Chawra, D., Zeitzer, J. 2022; 4 (4): 497-507


    The purpose of this study is to characterize the impact of the timing and duration of missing actigraphy data on interdaily stability (IS) and intradaily variability (IV) calculation. The performance of three missing data imputation methods (linear interpolation, mean time of day (ToD), and median ToD imputation) for estimating IV and IS was also tested. Week-long actigraphy records with no non-wear or missing timeseries data were masked with zeros or 'Not a Number' (NaN) across a range of timings and durations for single and multiple missing data bouts. IV and IS were calculated for true, masked, and imputed (i.e., linear interpolation, mean ToD and, median ToD imputation) timeseries data and used to generate Bland-Alman plots for each condition. Heatmaps were used to analyze the impact of timings and durations of and between bouts. Simulated missing data produced deviations in IV and IS for longer durations, midday crossings, and during similar timing on consecutive days. Median ToD imputation produced the least deviation among the imputation methods. Median ToD imputation is recommended to recapitulate IV and IS under missing data conditions for less than 24 h.

    View details for DOI 10.3390/clockssleep4040039

    View details for PubMedID 36278532

  • Bright Light During Wakefulness Improves Sleep Quality in Healthy Men: A Forced Desynchrony Study Under Dim and Bright Light (III) JOURNAL OF BIOLOGICAL RHYTHMS Lok, R., Woelders, T., Gordijn, M. M., van Koningsveld, M. J., Oberman, K., Fuhler, S. G., Beersma, D. M., Hut, R. A. 2022: 7487304221096910


    Under real-life conditions, increased light exposure during wakefulness seems associated with improved sleep quality, quantified as reduced time awake during bed time, increased time spent in non-rapid eye movement (NREM) sleep, or increased power of the electroencephalogram delta band (0.5-4 Hz). The causality of these important relationships and their dependency on circadian phase and/or time awake has not been studied in depth. To disentangle possible circadian and homeostatic interactions, we employed a forced desynchrony protocol under dim light (6 lux) and under bright light (1300 lux) during wakefulness. Our protocol consisted of a fast cycling sleep-wake schedule (13 h wakefulness-5 h sleep; 4 cycles), followed by 3 h recovery sleep in a within-subject cross-over design. Individuals (8 men) were equipped with 10 polysomnography electrodes. Subjective sleep quality was measured immediately after wakening with a questionnaire. Results indicated that circadian variation in delta power was only detected under dim light. Circadian variation in time in rapid eye movement (REM) sleep and wakefulness were uninfluenced by light. Prior light exposure increased accumulation of delta power and time in NREM sleep, while it decreased wakefulness, especially during the circadian wake phase (biological day). Subjective sleep quality scores showed that participants rated their sleep quality better after bright light exposure while sleeping when the circadian system promoted wakefulness. These results suggest that high environmental light intensity either increases sleep pressure buildup during wakefulness or prevents the occurrence of micro-sleep, leading to improved quality of subsequent sleep.

    View details for DOI 10.1177/07487304221096910

    View details for Web of Science ID 000815033900001

    View details for PubMedID 35730553

  • Bright Light Decreases Peripheral Skin Temperature in Healthy Men: A Forced Desynchrony Study Under Dim and Bright Light (II). Journal of biological rhythms Lok, R., Woelders, T., van Koningsveld, M. J., Oberman, K., Fuhler, S. G., Beersma, D. G., Hut, R. A. 2022: 7487304221096948


    Human thermoregulation is strictly regulated by the preoptic area of the hypothalamus, which is directly influenced by the suprachiasmatic nucleus (SCN). The main input pathway of the SCN is light. Here, thermoregulatory effects of light were assessed in humans in a forced desynchrony (FD) design. The FD experiment was performed in dim light (DL, 6lux) and bright white light (BL, 1300 lux) in 8 men in a semi-randomized within-subject design. A 4 * 18 h FD protocol (5h sleep, 13h wake) was applied, with continuous core body temperature (CBT) and skin temperature measurements at the forehead, clavicles, navel, palms, foot soles and toes. Skin temperature parameters indicated sleep-wake modulations as well as internal clock variations. All distal skin temperature parameters increased during sleep, when CBT decreased. Light significantly affected temperature levels during the wake phase, with decreased temperature measured at the forehead and toes and increased navel and clavicular skin temperatures. These effects persisted when the lights were turned off for sleep. Circadian amplitude of CBT and all skin temperature parameters decreased significantly during BL exposure. Circadian proximal skin temperatures cycled in phase with CBT, while distal skin temperatures cycled in anti-phase, confirming the idea that distal skin regions reflect heat dissipation and proximal regions approximate CBT. In general, we find that increased light intensity exposure may have decreased heat loss in humans, especially at times when the circadian system promotes sleep.

    View details for DOI 10.1177/07487304221096948

    View details for PubMedID 35723003

  • Bright Light Increases Alertness and Not Cortisol in Healthy Men: A Forced Desynchrony Study Under Dim and Bright Light (I). Journal of biological rhythms Lok, R., Woelders, T., van Koningsveld, M. J., Oberman, K., Fuhler, S. G., Beersma, D. G., Hut, R. A. 2022: 7487304221096945


    Light-induced improvements in alertness are more prominent during nighttime than during the day, suggesting that alerting effects of light may depend on internal clock time or wake duration. Relative contributions of both factors can be quantified using a forced desynchrony (FD) designs. FD designs have only been conducted under dim light conditions (<10lux) since light above this amount can induce non-uniform phase progression of the circadian pacemaker (also called relative coordination). This complicates the mathematical separation of circadian clock phase from homeostatic sleep pressure effects. Here we investigate alerting effects of light in a novel 4 * 18 h FD protocol (5h sleep, 13h wake) under dim (6lux) and bright light (1300lux) conditions. Hourly saliva samples (melatonin and cortisol assessment) and 2-hourly test sessions were used to assess effects of bright light on subjective and objective alertness (electroencephalography and performance). Results reveal (1) stable free-running cortisol rhythms with uniform phase progression under both light conditions, suggesting that FD designs can be conducted under bright light conditions (1300lux), (2) subjective alerting effects of light depend on elapsed time awake but not circadian clock phase, while (3) light consistently improves objective alertness independent of time awake or circadian clock phase. Reconstructing the daily time course by combining circadian clock phase and wake duration effects indicates that performance is improved during daytime, while subjective alertness remains unchanged. This suggests that high-intensity indoor lighting during the regular day might be beneficial for mental performance, even though this may not be perceived as such.

    View details for DOI 10.1177/07487304221096945

    View details for PubMedID 35686534

  • ISOLATED REM SLEEP BEHAVIOR DISORDER IS ASSOCIATED WITH 24-HOUR RHYTHM DISRUPTION Winer, J., Lok, R., Cahuas, A., Bueno, F., Poston, K., Mormino, E., Zeitzer, J., During, E. OXFORD UNIV PRESS INC. 2022: A125
  • Circadian photoreception: The impact of light on human circadian rhythms. Progress in brain research Zeitzer, J. M., Lok, R. 2022; 273 (1): 171-180


    Light is the preeminent external influence in determining the position of the internal circadian clock relative to the outside world. In this chapter, we discuss the different parameters of light that impact how it influences the human circadian clock. We detail how the timing (phase), intensity, duration and temporal structure, and spectral composition all can modulate the impact of light on both the timing of the circadian clock as well as its amplitude. The neurobiological underpinnings of the system are briefly discussed in the context of understanding how light can evoke its observed effects on the circadian clock.

    View details for DOI 10.1016/bs.pbr.2022.04.005

    View details for PubMedID 35940715

  • Impact of daytime spectral tuning on cognitive function. Journal of photochemistry and photobiology. B, Biology Lok, R., Joyce, D. S., Zeitzer, J. M. 2022; 230: 112439


    Light at night can improve alertness and cognition. Exposure to daytime light, however, has yielded less conclusive results. In addition to direct effects, daytime light may also mitigate the impact of nocturnal light exposure on alertness. To examine the impact of daytime lighting on daytime cognitive performance, and evening alertness, we studied nine healthy individuals using a within subject crossover design. On four visits, participants were exposed to one of four lighting conditions for 10h (dim fluorescent, room fluorescent, broad-spectrum LED, standard white LED; the latter three conditions were matched for 100lx) followed by an exposure to bright evening light. Cognitive performance, subjective and objective measures of alertness were regularly obtained. While daytime alertness was not impacted by light exposure, the broad-spectrum LED light improved several aspects of daytime cognition. The impact of evening light on alertness was not mitigated by the pre-exposure to different daytime lighting conditions. Results suggest that daytime exposure to white light with high melanopic efficacy has the potential to improve daytime cognitive function and that such improvements are likely to be direct rather than a consequence of light-induced changes in alertness.

    View details for DOI 10.1016/j.jphotobiol.2022.112439

    View details for PubMedID 35398657

  • Objective underpinnings of self-reported sleep quality in middle-aged and older adults: the importance of N2 and wakefulness. Biological psychology Lok, R., Chawra, D., Hon, F., Ha, M., Kaplan, K. A., Zeitzer, J. M. 2022: 108290


    STUDY OBJECTIVES: The measurable aspects of brain function (polysomnography, PSG) that are correlated with sleep satisfaction are poorly understood. Using recent developments in automated sleep scoring, which remove the within- and between-rater error associated with human scoring, we examine whether PSG measures are associated with sleep satisfaction.DESIGN AND SETTING: A single night of PSG data was compared to contemporaneously collected measures of sleep satisfaction with Random Forest regressions. Whole and partial night PSG data were scored using a novel machine learning algorithm.PARTICIPANTS: Community-dwelling adults (N=3,165) who participated in the Sleep Heart Health Study.INTERVENTIONS: None MEASUREMENTS AND RESULTS: Models explained 30% of sleep depth and 27% of sleep restfulness, with a similar top four predictors: minutes of N2 sleep, sleep efficiency, age, and minutes of wake after sleep onset (WASO). With increasing self-reported sleep quality, there was a progressive increase in N2 and decrease in WASO of similar magnitude, without systematic changes in N1, N3 or REM sleep. In comparing those with the best and worst self-reported sleep satisfaction, there was a range of approximately 30minutes more N2, 30minutes less WASO, an improvement of sleep efficiency of 7-8%, and an age span of 3-5 years. Examination of sleep most proximal to morning awakening revealed no greater explanatory power than the whole-night data set.CONCLUSIONS: Higher N2 and concomitant lower wake is associated with improved sleep satisfaction. Interventions that specifically target these may be suitable for improving the self-reported sleep experience.

    View details for DOI 10.1016/j.biopsycho.2022.108290

    View details for PubMedID 35192907

  • Bright Light Decreases Peripheral Skin Temperature in Healthy Men: A Forced Desynchrony Study Under Dim and Bright Light (II) Journal of Biological Rhythms Lok, R. 2022
  • Physiological correlates of the Epworth Sleepiness Scale reveal different dimensions of daytime sleepiness. Sleep advances : a journal of the Sleep Research Society Lok, R., Zeitzer, J. M. 2021; 2 (1): zpab008


    The Epworth Sleepiness Scale is commonly used to examine self-reported daytime sleepiness in clinical populations; the physiologic correlates of this scale, however, are not well understood. Furthermore, how well this scale correlates with parallel objective and self-reported concepts of daytime sleepiness is not well described. As such, we used machine learning algorithms to examine the association between Epworth Sleepiness Scale scores and 55 sleep and medical variables in the Sleep Heart Health Study (N = 2105). Secondary analyses examined data stratified by age and gender and the relationship between the Epworth and other measures of daytime sleepiness. Analyses of the main data set resulted in low explained variance (7.15%-10.0%), with self-reported frequency of not getting enough sleep as most important predictor (10.3%-13.9% of the model variance). Stratification by neither age nor gender significantly improved explained variance. Cross-correlational analysis revealed low correlation of other daytime sleepiness measures to Epworth scores. We find that Epworth scores are not well explained by habitual or polysomnographic sleep values, or other biomedical characteristics. These analyses indicate that there are different, potentially orthogonal dimensions of the concept of "daytime sleepiness" that may be driven by different aspects of sleep physiology. As the physiologic correlates of the Epworth Sleepiness Scale remain to be elucidated, interpretation of the clinical meaning of these scores should be done with caution.

    View details for DOI 10.1093/sleepadvances/zpab008

    View details for PubMedID 34250482

  • A Temporal Threshold for Distinguishing Off-Wrist from Inactivity Periods: A Retrospective Actigraphy Analysis. Clocks & sleep Lok, R., Zeitzer, J. M. 2020; 2 (4): 466–72


    (1) Background. To facilitate accurate actigraphy data analysis, inactive periods have to be distinguished from periods during which the device is not being worn. The current analysis investigates the degree to which off-wrist and inactive periods can be automatically identified. (2) Methods. In total, 125 actigraphy records were manually scored for 'off-wrist' and 'inactivity' (99 collected with the Motionlogger AMI, 26 (sampling frequency of 60 (n = 20) and 120 (n = 6) s) with the Philips Actiwatch 2.) Data were plotted with cumulative frequency percentage and analyzed with receiver operating characteristic curves. To confirm findings, the thresholds determined in a subset of the Motionlogger dataset (n = 74) were tested in the remaining dataset (n = 25). (3) Results. Inactivity data lasted shorter than off-wrist periods, with 95% of inactive events being shorter than 11 min (Motionlogger), 20 min (Actiwatch 2; 60 s epochs) or 30 min (Actiwatch 2; 120 s epochs), correctly identifying 35, 92 or 66% of the off-wrist periods. The optimal accurate detection of both inactive and off-wrist periods for the Motionlogger was 3 min (Youden's Index (J) = 0.37), while it was 18 (J = 0.89) and 16 min (J = 0.81) for the Actiwatch 2 (60 and 120 s epochs, respectively). The thresholds as determined in the subset of the Motionlogger dataset showed similar results in the remaining dataset. (4) Conclusion. Off-wrist periods can be automatically identified from inactivity data based on a temporal threshold. Depending on the goal of the analysis, a threshold can be chosen to favor inactivity data's inclusion or accurate off-wrist detection.

    View details for DOI 10.3390/clockssleep2040034

    View details for PubMedID 33198122

  • Gold, silver or bronze: circadian variation strongly affects performance in Olympic athletes SCIENTIFIC REPORTS Lok, R., Zerbini, G., Gordijn, M. M., Beersma, D. M., Hut, R. A. 2020; 10 (1): 16088


    The circadian system affects physiological, psychological, and molecular mechanisms in the body, resulting in varying physical performance over the day. The timing and relative size of these effects are important for optimizing sport performance. In this study, Olympic swim times (from 2004 to 2016) were used to determine time-of-day and circadian effects under maximal motivational conditions. Data of athletes who made it to the finals (N = 144, 72 female) were included and normalized on individual levels based on the average swim times over race types (heat, semifinal, and final) per individual for each stroke, distance and Olympic venue. Normalized swim times were analyzed with a linear mixed model and a sine fitted model. Swim performance was better during finals as compared to semi-finals and heats. Performance was strongly affected by time-of-day, showing fastest swim times in the late afternoon around 17:12 h, indicating 0.32% improved performance relative to 08:00 h. This study reveals clear effects of time-of-day on physical performance in Olympic athletes. The time-of-day effect is large, and exceeds the time difference between gold and silver medal in 40%, silver and bronze medal in 64%, and bronze or no medal in 61% of the finals.

    View details for DOI 10.1038/s41598-020-72573-8

    View details for Web of Science ID 000577480800001

    View details for PubMedID 33033271

    View details for PubMedCentralID PMC7544825

  • Daytime melatonin and light independently affect human alertness and body temperature JOURNAL OF PINEAL RESEARCH Lok, R., van Koningsveld, M. J., Gordijn, M. M., Beersma, D. M., Hut, R. A. 2019; 67 (1): e12583


    Light significantly improves alertness during the night (Cajochen, Sleep Med Rev, 11, 2007 and 453; Ruger et al., AJP Regul Integr Comp Physiol, 290, 2005 and R1413), but results are less conclusive at daytime (Lok et al., J Biol Rhythms, 33, 2018 and 589). Melatonin and core body temperature levels at those times of day may contribute to differences in alerting effects of light. In this experiment, the combined effect of daytime exogenous melatonin administration and light intensity on alertness, body temperature, and skin temperature was studied. The goal was to assess whether (a) alerting effects of light are melatonin dependent, (b) soporific effects of melatonin are mediated via the thermoregulatory system, and (c) light can improve alertness after melatonin-induced sleepiness during daytime. 10 subjects (5 females, 5 males) received melatonin (5 mg) in dim (10 lux) and, on a separate occasion, in bright polychromatic white light (2000 lux). In addition, they received placebo both under dim and bright light conditions. Subjects participated in all four conditions in a balanced order, yielding a balanced within-subject design, lasting from noon to 04:00 pm. Alertness and performance were assessed half hourly, while body temperature and skin temperature were measured continuously. Saliva samples to detect melatonin concentrations were collected half hourly. Melatonin administration increased melatonin concentrations in all subjects. Subjective sleepiness and distal skin temperature increased after melatonin ingestion. Bright light exposure after melatonin administration did not change subjective alertness scores, but body temperature and proximal skin temperature increased, while distal skin temperature decreased. Light exposure did not significantly affect these parameters in the placebo condition. These results indicate that (a) exogenous melatonin administration during daytime increases subjective sleepiness, confirming a role for melatonin in sleepiness regulation, (b) bright light exposure after melatonin ingestion significantly affected thermoregulatory parameters without altering subjective sleepiness, therefore temperature changes seem nonessential for melatonin-induced sleepiness, (c) subjective sleepiness was increased by melatonin ingestion, but bright light administration was not able to improve melatonin-induced sleepiness feelings nor performance. Other (physiological) factors may therefore contribute to differences in alerting effects of light during daytime and nighttime.

    View details for DOI 10.1111/jpi.12583

    View details for Web of Science ID 000474798100005

    View details for PubMedID 31033013

    View details for PubMedCentralID PMC6767594

  • Light, Alertness, and Alerting Effects of White Light: A Literature Overview JOURNAL OF BIOLOGICAL RHYTHMS Lok, R., Smolders, K. J., Beersma, D. M., de Kort, Y. W. 2018; 33 (6): 589-601


    Light is known to elicit non-image-forming responses, such as effects on alertness. This has been reported especially during light exposure at night. Nighttime results might not be translatable to the day. This article aims to provide an overview of (1) neural mechanisms regulating alertness, (2) ways of measuring and quantifying alertness, and (3) the current literature specifically regarding effects of different intensities of white light on various measures and correlates of alertness during the daytime. In general, the present literature provides inconclusive results on alerting effects of the intensity of white light during daytime, particularly for objective measures and correlates of alertness. However, the various research paradigms employed in earlier studies differed substantially, and most studies tested only a limited set of lighting conditions. Therefore, the alerting potential of exposure to more intense white light should be investigated in a systematic, dose-dependent manner with multiple correlates of alertness and within one experimental paradigm over the course of day.

    View details for DOI 10.1177/0748730418796443

    View details for Web of Science ID 000450159000002

    View details for PubMedID 30191746

    View details for PubMedCentralID PMC6236641

  • White Light During Daytime Does Not Improve Alertness in Well-rested Individuals JOURNAL OF BIOLOGICAL RHYTHMS Lok, R., Woelders, T., Gordijn, M. M., Hut, R. A., Beersma, D. M. 2018; 33 (6): 637-648


    Broad-spectrum light applied during the night has been shown to affect alertness in a dose-dependent manner. The goal of this experiment was to investigate whether a similar relationship could be established for light exposure during daytime. Fifty healthy participants were subjected to a paradigm (0730-1730 h) in which they were intermittently exposed to 1.5 h of dim light (<10 lux) and 1 h of experimental light (24-2000 lux). The same intensity of experimental light was used throughout the day, resulting in groups of 10 subjects per intensity. Alertness was assessed with subjective and multiple objective measures. A significant effect of time of day was found in all parameters of alertness ( p < 0.05). Significant dose-response relationships between light intensity and alertness during the day could be determined in a few of the parameters of alertness at some times of the day; however, none survived correction for multiple testing. We conclude that artificial light applied during daytime at intensities up to 2000 lux does not elicit significant improvements in alertness in non-sleep-deprived subjects.

    View details for DOI 10.1177/0748730418796036

    View details for Web of Science ID 000450159000006

    View details for PubMedID 30191761

    View details for PubMedCentralID PMC6236585