Light and the biological clock

24h rhythms

Virtually all places on Earth are exposed to an endless sequence of days and nights. This leads to an alternation between light and darkness that occurs every 24 hours. Predictable changes accompany the 24 hours day such as variations in daylight and temperature. In response to these predictable changes, a biological clock has evolved practically in all organisms including humans. In humans, this biological clock is located in the brain, in an area called the Nucleus Suprachiasmaticus. It consists of about 20 000 neurons that together generate a rhythm of approximately 24 hours. The main environmental cue that allows the biological clock to keep entrained to the exact external 24 hours day are the light/dark changes. The biological clock allows organisms to anticipate (instead of merely react to) the daily changes in the environment. Anticipation occurs because of changes in physiology and behavior; the so-called biological rhythms. These rhythms represent an internal notion of time. The field that studies these rhythms is known as chronobiology. In particular the circadian rhythms refer to those biological rhythms with a period close to the 24 hours.

 

Sleep-wake rhythm

The 24 hours sleep-wake rhythm is the most obvious circadian behavior in humans. Sleep is a complex phenomenon and its regulation goes beyond the circadian system. As a matter of fact, the alternation of sleep and wakefulness is thought to be the result of an interaction between homeostatic and circadian processes. The homeostatic mechanism keeps track of how long we have been awake (and asleep); being awake for a longer time means that we will sleep deeper. The circadian process determines the optimal time for sleep.  Together these two processes interact in order to consolidate sleep at night and optimize waking during the day (Daan et al 1984, Beersma & Gordijn 2007, Foster 2010).

 

Social and biological clocks

The biological (or so-called circadian) clock takes advantage of the environmental predictability in order to allow physiological processes to occur at the right time. Although the master circadian pacemaker is located in the brain, in the nucleus suprachiasmaticus, various organs in our body have clocks as well. The clocks in organs, such as for instance the liver, take care that each part of the body is optimally prepared for its function at a specific time of day. So is the liver entrained to our rhythmic food intake, and is our blood pressure raised prior to waking up and the start of activity (Foster 2010).

Optimal synchronization between internal rhythms and the outside world is important for healthy sleep and wellbeing and it has been shown in free living animals that fitness is highest in those animals that are best adapted to the outside rhythmic world (Spoelstra et al 2015). Nowadays, the presence of electricity and artificial light has enormously influenced the predictability of the environment: humans are able to light up their nights and by spending most time of the day under indoor lighting conditions their day is not that different from the night anymore.  Moreover, the 24/7 society challenges our sleep necessities and internal timing system; our social clock often overrules the biological one. As a consequence, circadian rhythms- and sleep disruption have been linked to a broad range of pathologies; for example poor vigilance and memory, depression, and metabolic abnormalities (Foster 2010).

 

Melatonin, the hormone of darkness

Melatonin is produced in the pineal gland, a part of our brain, and its rhythmic synthesis is under direct regulation of the biological clock.  During daytime the biological clock inhibits melatonin production while this inhibition is relieved at nighttime (Simonneaux & Ribelayga, 2003). Hence, the melatonin rhythm signals the biological night; it starts to rise in the evening and declines in the early morning. The rhythm of melatonin has been described as one of the most robust outputs of the biological clock in humans (Klerman et al., 2002) and is widely used in chronobiological human studies as a phase marker of the clock. The rhythm of melatonin can be shifted by light (Lewy et al., 1985) as well as it can be acutely suppressed by it (Lewy et al., 1980). In particular, the melatonin suppression by light test is often used to describe the sensitivity of the non-image forming system in humans (i.e. the effects that light has on our physiology and behavior and that are not related to vision).

There is a relationship between the rhythm of melatonin and the rhythm of sleep: sleep is better if it starts shortly after the endogenous rise of melatonin in the evening. Melatonin also feeds back to the biological clock, in other words melatonin administration at a certain time of day will shift the clock. A direct role of melatonin on sleep itself is unlikely; in nocturnal animals melatonin is high during the active phase. In that sense it is not wise to call melatonin a ‘sleep hormone’ but rather the hormone of darkness.

 

Shifting the clock with light

Light in the morning at the appropriate time can be used to shift the biological clock to an earlier phase light in the evening shifts the clock to a later phase. The timing dependent shifts of the clock are described by a phase response curve (Khalsa et al 2003). Although this is the most complete phase response curve to single pulses of bright white light, it is derived from a highly controlled laboratory experiment with light pulses of 6.7h. Actually recent studies show that such a long light exposure is not necessary to induce phase shifts of the clock. Appropriately timed light exposure to either white or blue light of sufficient intensity with a duration of 30 min to 1h is enough to induce phase shifts of the clock, even in a home setting (Geerdink et al, 2016; St Hilaire et al 2012).

 

Dawn simulation to improve waking up

A special way of light exposure is applied with the use of alarm clocks with light. These so called wake up lights show a gradual increase of light intensity in the last half hour before waking up. Filtered light is entering the eye through closed eyelids. Several experiments have shown that waking up with this early morning light decreases the amount of sleep inertia, attenuate sudden cardiac changes at waking up, and improves wellbeing and cognition during the day (Gabel et al 2013, Giménez et al 2010, Van de Werken et al 2010, Viola et al. 2015), but it is not inducing a shift of the clock (Gabel et al 2013, Giménez et al 2010).

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