Sleep and Homeostasis

Homeostasis: Sleep and Health Sleep and Health How Sleep Homeostasis Helps to Maintain Human Health This booklet is designed to answer questions about how the human body's cycle of sleep and wakefulness, and to explain a few ways that sleep contributes to overall health. The Sleep System Sleep is governed by internal changes in the body that work together to produce healthy patterns of sleep. Over the course of our waking hours, our homeostatic sleep drive strengthens.

The level of brain activity is associated with our patterns of sleep and wakefulness. Sleep theory suggests that adenosine is produced when active and alert brain cells use energy. Sleep drive and adenosine increase in concert 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 that we sleep varies according to the quality and quantity of sleep that we attained earlier in the day or night.

While the sleep drive can be masked -- by consuming caffeine or striving to increase physical activity, for instance -- the only true way to reduce the sleep drive is by sleeping. Our Internal Biological Clock Our biological clock regulates many different daily cycles from a small set of neurons that are located deep within the brain. Our daily sleep / wake cycle is characterized by a relatively steady state of alertness for 16 hours of a typical day ("Division of Sleep Medicine," 2007).

Our biological clock is highly synchronized to our sleep / wake cycle by a circadian alerting system. The chemical and hormonal alerting system grows stronger with every hour that we are awake, and simultaneously serves to oppose the sleep drive, which is also always present throughout the day ("Division of Sleep Medicine," 2007). When the alerting signal of the biological clock falls below a certain level, the load of the sleep drive overcomes the circadian alerting system and allows the brain to fall asleep ("Division of Sleep Medicine," 2007).

While You Were Sleeping The first four hours or so after we fall asleep, our sleep drive is fairly strong, which makes it easy to stay asleep. But soon the internal clock begins to send signals to the parts of the brain that govern wakefulness (Saper, et al., 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 when we are asleep and when we are awake.

At some point in the middle of the afternoon, the alerting signal diminishes for an hour or two. But the sleep drive continues to strengthen during this time, so people become sleepy or less alert (Saper, et al., 2005). It is not just serendipity 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 P.M. To 9:00 P.M.

-- which makes it nearly impossible to fall asleep during that time (Saper, et al., 2005). This is why "sleep drive and sleepiness are not synonymous" (Saper, et al., 2005). Sleep Loss When people sleep fewer hours than they need, their body may respond by changing glucose metabolism in a way that increases insulin levels (Knutson, 2007). This can increase their risk of the development of diabetes. Sleep loss is also associated with changes in appetite that can lead to eating more food and gaining weight (Knutson, 2007).

Research conducted to study the effect of sleep loss on the human body has shown associations between short sleep duration (sleep loss) and increased risk of developing diabetes and obesity (Knutson, 2007). Figure 1. Healthy amounts of sleep help to balance energy intake and energy expenditure. SLEEP LOSS Increased appetite More time to eat Unhealthy food choice Reduced energy expenditure Reduced physical activity Altered glucose metabolism Disruption of Homeostasis Unlike cats and dogs, creatures known to take short naps throughout the day, healthy human experience consolidated periods of wakefulness and sleep.

The synchronization of the sleep drive and circadian alerting system make it possible for an individual to sleep and wake on a schedule that is more or less in harmony with other people in their environment. However, people who work night shifts or fly across time zones find that these activities disrupt or desynchronize the patterns of sleeping and waking that they have become accustomed under more typical circumstances.

The circadian rhythms are disrupted to the extent that it becomes impossible to be alert and to sleep when it is desirable in the changed circumstances. Although the brain regulates the transitions between sleep and wakefulness, the ability to achieve healthful sleep / wake cycles is also strongly influenced by external stimuli light and temperature, and by the consumption of caffeine and alcohol (Thakkar & Sahota, 2014). Sleep Cycles and the Lifecycle The circadian rhythm experiences changes during adolescence causing most teenagers to experiences a delay in their sleep phase.

The natural peak of alertness that occurs just before bedtime for most people seems to naturally extend for young people in their teenage years. This can make it difficult for teenagers to fall asleep before 11:00 P.M. ("National Sleep Foundation," 2014).