On the surface of it, the timing of falling asleep seems fairly easy to understand. Once you have been awake for a certain period of time, you become tired and eventually go to sleep. The technical term for increasing sleepiness over time is the homeostatic sleep drive, as I explained in a previous post. However, there is another system in the body which helps to regulate sleep and wake timing. This is referred to as the body clock, which governs your circadian rhythmn. The body clock regulated multiple systems in the body even down to the functioning of individual cells.
In the scientific literature, this system is described as the two process model of sleep. The role of the circadian drive is to keep you awake in the early evening. Under perfect conditions, the circadian drive slackens off in the late evening and then, during the night, plays a role in promoting sleep continuity as your sleep drive is reducing. Thus, the sleep drive and the body clock work together to promote both a sustained period of alertness and a sustained period of sleep (16 hours awake and 8 hours sleeping in adults). I would note that the circadian system is quite complex and sends signals which promote both sleep and wakefulness at various times of the day. Melatonin is one of the chemicals secreted in the brain to induce sleepiness as part of the biological clock.
As children enter puberty, the timing of the wakefulness signal shifts later, resulting in later times of sleep onset. Exposure to blue light frequencies (e.g. from computer, phone, television, or tablet screens) can shift this even later. Add in earlier school start times, and you have a recipe for chronic sleep deprivation in teenagers . Some authors have described the lack of synchronization of the body clock and school schedules as “social jet lag“, where teens only are sleeping in their physiologic sleep period on weekends, and feeling out of it and ill at ease during the week.
Although we think of an “evening preference” (late to bed, late to rise) occurring in teenagers, younger children can develop a preference for staying up late as well.
You can adjust the timing of the body clock, by the application of light or melatonin. It is tricky, however, as the timing of these interventions determines how strong the effect is, and even the direction of the effect. In general, light is the strongest stimulus for body clock adjustments. Light exposure in the evening will “wake you up” and shift your body clock later. Light exposure in the morning will move your body clock earlier. Exercise has similar effects, so teenagers and adults with insomnia may benefit from morning exercise outside, and avoidance of nocturnal exercise or screen time before bedtime. Melatonin in the early evenings at small doses may move the body clock earlier and help with sleep onset.
For more information, here is a great interactive diagram which depicts the relationship between these two systems in a dynamic manner. Note that the sleep drive is depicted upside down from in the diagrams above.
On a side note, actual jet lag is the result of moving across time zones while your body clock stays the same. This can result in severe daytime sleepiness and nocturnal insomnia until your body clock habituates. Here’s an article which provides some guidance on how to combat jet lag.
Do you have any questions related to the functioning of the body clock? Have you tried to adjust your sleep wake schedule (or that of your child) via the application of light or melatonin?