Almost every function of the body is influenced by a daily biological clock known as the circadian rhythm. There are several systems that keep their own rhythms, but the master clock that controls them is located in a very small structure at the base of the brain known as the hypothalamus. Within the hypothalamus is an even smaller structure comprised of no more than 20 thousand cells. This site is called the suprachiasmatic nucleus (SCN). Considering that the brain contain 100 billion cells, it is amazing that so few neurons can control the whole body.
The sleep-wake cycle is controlled by several mechanisms, but ultimately, it is the hypothalamus master clock that determines rhythmicity. There is one sleep control that is not reliant on the SCN. This is called sleep-pressure. The body likes for things to remain in a steady state, and has mechanisms to maintain homeostasis (same state). To accomplish this with sleep, one usually becomes increasingly tired and sleepy the longer they remain awake. This sleep-pressure prevents one from building up a sleep debt and becoming sleep-deprived. The remaining mechanisms that help control the sleep-wake circadian rhythm rely on the master clock to remain synchronized.
The master clock controls one’s level of alertness and sleepiness following a circadian pattern. In adults, alertness during the day ebbs and sleepiness occurs between 1:00-3:00 PM. It is no wonder that some cultures traditionally take a siesta in the early afternoon. At night, sleep-drive peaks between 2:00-4:00 AM. Sleep is also regulated by a hormone called melatonin. Melatonin is secreted by the Pineal Gland, which is a small structure located in the rear of the brain. Melatonin promotes sleep and causes a decrease in body temperature. The Pineal Gland releases melatonin all day long, but its levels increase substantially about two hours before bedtime. Once asleep, melatonin levels continue to increase and peak at between 2:00-3:00 AM. However, there must be some way to turn off the Pineal Gland upon awakening in the morning in order to inhibit further melatonin release. As strange as it may seem, the way the body communicates this is to the Pineal Gland is through the retinal nerve of the eye. When someone sees light first thing in the morning, the stimulation of the retina sends a message via the retinal nerve to cause the Pineal Gland to reduce its activity.
Light exposure also serves to reset the master clock every day and synchronize it to a regular 24 hour cycle. Without this reset, the free-running circadian rhythm in most people is actually 25 hours or more. This becomes problematic with people who are totally blind. Without the retinal cue, they cannot keep a 24 hour cycle. This condition has come to be known as non-24 or N24. Sleep is delayed by one hour every day, and does not allow for the whole body to remain synchronized. It is difficult for these people to function, as they are chronically sleep-deprived. This can cause excessive daytime sleepiness, fatigue, depression, difficulty concentrating, and memory impairments. The ideal amount of sleep for adults is between 7-9 hours. Adolescents need more and find it difficult to wake up in the morning. It is best to go to bed and wake up at the same time every day. This produces a consistent sleep-wake cycle and the synchronization of the master-clock to insure that there be a stable circadian rhythm around which the rest of the body depends on to function properly. Good sleep hygiene will promote both physical and mental health.
Blog posts are written by Shore House members and staff.