Dawn, light at night and the clock:impact on human alertness, performance and physiology

In onze huidige 24 uur maatschappij zijn er steeds meer mensen wakker op tijdstippen waarop ons lichaam liever zou willen slapen. Werken tijdens de nacht, laat opblijven ’s nachts voor een feest, vroeg opstaan in de ochtend voor werk, of door tijdzones vliegen, is veelvoorkomend gedrag dat in conflict staat met onze lichamelijke voorkeur voor slaap/waak verdeling over de 24 uur. Onze biologische klok in de hersenen, op tijd gezet door lichtinformatie van met name de zon, regelt onze 24 uurs ritmiek in fysiologie en gedrag. Het kan dus zijn dat je lichaam gereed gemaakt wordt om te slapen, maar het midden op de dag is omdat je net van Amsterdam naar New York gevlogen bent. Individuele verschillen in afstemming en periode van de biologische klok zorgen er voor dat mensen verschillen in de tijd dat zij het liefst slapen of opstaan. Dit maakt bepaalde mensen minder of meer geschikt voor werk op bepaalde tijdstippen van de dag. Niet iedereen zal werken in nachtdienst of in de vroege ochtend kunnen verdragen. Hierdoor zullen veel mensen kiezen voor werk dat uitgevoerd wordt op tijdstippen die het beste passen bij hun voorkeur voor de tijdstippen waarop zij wakker willen zijn. Desalniettemin zijn er veel mensen die in nachtdiensten werken of doordeweeks vroeger op moeten staan dan zij zouden willen. Wakker zijn op momenten waarop men liever zou slapen kan gevolgen hebben voor de alertheid, iets wat de kans op fouten en op ongelukken verhoogt. Met behulp van licht kan de alertheid worden verhoogd. Met name blauw licht, licht met een korte golflengte, is effectief in het verhogen van een gevoel van activatie en het verminderen van slaperigheid. Vooral tijdens werken in de nacht kan licht een belangrijke rol spelen in het wakker en alert houden van mensen. Hierdoor kan de productiviteit hoger komen te liggen en vermindert de kans op ongelukken/fouten veroorzaakt door vermoeidheid.

Short-wavelength attenuated polychromatic white light during work at night: limited melatonin suppression without substantial decline of alertness

Exposure to light at night increases alertness, but light at night (especially short-wavelength light) also disrupts nocturnal physiology. Such disruption is thought to underlie medical problems for which shiftworkers have increased risk. In 33 male subjects we investigated whether short-wavelength attenuated polychromatic white light (<530nm filtered out) at night preserves dim light melatonin levels and whether it induces similar skin temperature, alertness, and performance levels as under full-spectrum light. All 33 subjects participated in random order during three nights (at least 1 wk apart) either under dim light (3 lux), short-wavelength attenuated polychromatic white light (193 lux), or full-spectrum light (256 lux). Hourly saliva samples for melatonin analysis were collected along with continuous measurements of skin temperature. Subjective sleepiness and activation were assessed via repeated questionnaires and performance was assessed by the accuracy and speed of an addition task. Our results show that short-wavelength attenuated polychromatic white light only marginally (6%) suppressed salivary melatonin. Average distal-to-proximal skin temperature gradient (DPG) and its pattern over time remained similar under short-wavelength attenuated polychromatic white light compared with dim light. Subjects performed equally well on an addition task under short-wavelength attenuated polychromatic white light compared with full-spectrum light. Although subjective ratings of activation were lower under short-wavelength attenuated polychromatic white light compared with full-spectrum light, subjective sleepiness was not increased. Short-wavelength attenuated polychromatic white light at night has some advantages over bright light. It hardly suppresses melatonin concentrations, whereas performance is similar to the bright light condition. Yet, alertness is slightly reduced as compared with bright light, and DPG shows similarity to the dim light condition, which is a physiological sign of reduced alertness. Short-wavelength attenuated polychromatic white light might therefore not be advisable in work settings that require high levels of alertness.

Effects of Artificial Dawn on Subjective Ratings of Sleep Inertia and Dim Light Melatonin Onset

The timing of work and social requirements has a negative impact on performance and well-being of a significant proportion of the population in our modern society due to a phenomenon known as social jetlag. During workdays, in the early morning, late chronotypes, in particular, suffer from a combination of a nonoptimal circadian phase and sleep deprivation. Sleep inertia, a transient period of lowered arousal after awakening, therefore, becomes more severe. In the present home study, the authors tested whether the use of an alarm clock with artificial dawn could reduce complaints of sleep inertia in people having difficulties in waking up early. The authors also examined whether these improvements were accompanied by a shift in the melatonin rhythm. Two studies were performed: Study 1: three conditions (0, 50, and 250 lux) and Study 2: two conditions (0 lux and self-selected dawn-light intensity). Each condition lasted 2 weeks. In both studies, the use of the artificial dawn resulted in a significant reduction of sleep inertia complaints. However, no significant shift in the onset of melatonin was observed after 2 weeks of using the artificial dawn of 250 lux or 50 lux compared to the control condition. A multilevel analysis revealed that only the presence of the artificial dawn, rather than shift in the dim light melatonin onset or timing of sleep offset, is related to the observed reduction of sleep inertia complaints. Mechanisms other than shift of circadian rhythms are needed to explain the positive results on sleep inertia of waking up with a dawn signal.

The biological clock modulates the human cortisol response in a multiplicative fashion

Human cortisol levels follow a clear circadian rhythm. We investigated the contribution of alternation of sleep and wakefulness and the circadian clock, using forced desynchrony. Cortisol levels were best described by a multiplication of a circadian and a wake-time component. The human cortisol response is modulated by circadian phase. Exposure to stress at an unnatural phase, as in shift work, is predicted to result in abnormal cortisol levels. Health of shift workers may therefore improve when stress is reduced at times when the clock produces high stress sensitivity