This paper examines how the human body regulates sleep through two interacting systems: the homeostatic sleep drive and the circadian alerting system. It explains the role of adenosine in building sleep pressure, how the biological clock synchronizes wakefulness and rest, and what occurs physiologically during sleep cycles. The paper also addresses the health consequences of sleep loss — including increased risk of diabetes and obesity — as well as the effects of disruptions such as shift work and jet lag. Finally, it considers how sleep needs and circadian patterns change across the lifespan, with particular attention to adolescent sleep deprivation.
Sleep is governed by internal changes in the body that work together to produce healthy patterns of rest. Over the course of our waking hours, our homeostatic sleep drive gradually strengthens. The level of brain activity is closely associated with our patterns of sleep and wakefulness. Sleep theory suggests that adenosine is produced when active, alert brain cells consume energy. Sleep drive and adenosine increase together during wakefulness, and the level of adenosine in the brain dissipates as the sleep drive lessens and we enter a stage of wakefulness.
How deeply we sleep, or the length of time we sleep, varies according to the quality and quantity of sleep attained earlier in the day or night. While the sleep drive can be temporarily masked — by consuming caffeine or increasing physical activity, for instance — the only true way to reduce the sleep drive is by sleeping.
Our biological clock regulates many different daily cycles from a small set of neurons located deep within the brain. Our daily sleep/wake cycle is characterized by a relatively steady state of alertness for approximately 16 hours of a typical day (Division of Sleep Medicine, 2007). The biological clock is highly synchronized to our sleep/wake cycle by a circadian alerting system. This chemical and hormonal alerting system grows stronger with every hour that we are awake, simultaneously opposing the sleep drive, which is also present throughout the day (Division of Sleep Medicine, 2007).
When the alerting signal of the biological clock falls below a certain level, the accumulated sleep drive overcomes the circadian alerting system and allows the brain to fall asleep (Division of Sleep Medicine, 2007). The circadian rhythm thus acts as a counterbalancing force that keeps us alert during the day and permits consolidated sleep at night.
During the first four hours or so after we fall asleep, our sleep drive is fairly strong, making it easy to stay asleep. But soon the internal clock begins to send signals to the parts of the brain that govern wakefulness (Saper, Cano, & Scammell, 2005). The balance slowly tips, and the sleeping person begins to move toward a wakeful state. In this way, the synchronized sleep drive and the circadian system interact to provide consolidated periods of sleep and wakefulness.
At some point in the middle of the afternoon, the alerting signal diminishes for an hour or two. Because the sleep drive continues to strengthen during this time, people become sleepy or less alert (Saper et al., 2005). It is no coincidence that many cultures have adapted to this brief lull in alertness by instituting siestas or daily mid-afternoon naps. As evening falls and bedtime approaches, the alerting signal rises to its highest level — often around 8:00 to 9:00 P.M. — which makes it nearly impossible to fall asleep during that window (Saper et al., 2005). This is why "sleep drive and sleepiness are not synonymous" (Saper et al., 2005).
"Sleep deprivation linked to diabetes and obesity"
"Shift work, jet lag, and external sleep disruptors"
"Teen sleep delays and older adult sleep changes"
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