Imagine sitting out in space watching the earth gently rotate eastward on its axis, providing daily sunsets and dawns to all us 7.6 billion earthlings.
In the sunlit surface of earth, nearly everybody is awake, and on the dark backside, everyone is asleep – or trying to sleep – except for the nightshift workers (who are struggling to stay awake). Imagine observing what happens as the earth rotates: When the sun rises, people wake up. and when the sun sets, they start sleeping. Why is that? Of course, it is more practical to be awake when there is daylight, but that’s not the whole story.
Humans as well as animals have an inner timekeeping system that guides us to sleep at night and be awake and alert in daytime. This system is located in the brain and includes the master clock (the suprachiasmatic nuclei or SCN) and the pineal gland, which secretes the hormone melatonin, helping us fall and stay asleep. The neurons in the SCN have a rhythm of approximately but not exactly 24 hours, untied to day and night. In the absence of an external day-night cycle, they cycle on their own with a rhythm of more or less 24 h 12 min, depending on the individual. The SCN triggers pineal melatonin to rise in the evening, helping make us sleepy, and subsides in the morning, helping us wake up.
If you were to live in constantly dark or dim surroundings, your sleep-wake cycle (and other rhythms) would drift later about 12 minutes a day (sometimes much more), following the internal SCN rhythm’s output signal to the pineal gland and its melatonin secretion pattern. But when you experience daylight or adequate indoor lighting, the sleep-wake cycle is surprisingly close to 24 hours. This adjustment is possible because we also have a system that secures a connection from the outer environment to our inner physiology. These outer signals serve as solar-cycle timekeepers (from the original German, Zeitgeber, or “time giver”). The daily light-dark cycle is by far the strongest zeitgeber, but daily cycles of social contact, food intake, and exercise can also cue the SCN into synchrony, controlling its inherent drifting pattern.
Light acts as a zeitgeber by stimulating specialized receptors in the retina that are very sensitive to blue light. In turn, these signals reach the SCN’s timekeeping system in the brain, thereby entraining our rhythms and sleep to the external 24-hour solar cycle. This entrainment process works in an ingenious way, such that light in the period after waking up advances the sleep-wake rhythm to better match the external day, whereas light before sleep delays the sleep-wake rhythm, which can cause you to fall asleep later. (This can feel like insomnia if you want to keep a normal bedtime.) If you have gotten out of sync with the external light-dark cycle after flying to a different time zone, the shifted zeitgebers will adjust your sleep in a matter of days whether you traveled east (earlier) or west (later).
The entrainment system needs a certain amount of light to adjust and keep a regular sleep-wake cycle (and sense of wellbeing). In modern day life, where many of us live almost all day under electric lighting and behind light-filtering window glass, there is a real risk that we do not get enough light input from the solar cycle to stabilize our rhythms and sleep. That’s why it is important for all of us to get an adequate and consistent amount of daytime light stimulation.
New ideas are afoot on how we can construct our built environment, in cities with limited light availability, to make it possible to obtain more daylight throughout the day and across the seasons. These architectural innovations will provide more access to daylight, in combination with dynamic electrical lighting simulation of the daylight cycle.
Klaus Martiny MD, DMSc, PhD, is a member of the Board of Directors of the Center for Environmental Therapeutics, and a past president of the Society for Light Treatment and Biological Rhythms. He is Professor of Clinical Psychiatry at the University of Copenhagen, Senior Consultant at Psychiatric Center Copenhagen, and founder of New Interventions in Depression (the NID-Group). His research focuses on new treatment methods for depression, emphasizing chronobiological techniques.