Knowing how circadian rhythms regulate sleep and activity helps us better understand why we sometimes have sleep problems, and what we can do to prevent circadian rhythm problems from happening.
It doesn't take a sleep expert to know that the most powerful influences over sleep are light and darkness. Through eons of time, people have awakened to morning light and fallen asleep in evening darkness. This cycle seems natural; we feel energetic in sunlight and yawn and become lethargic at night. Those of us on different schedules have to fight to stay awake and struggle to sleep.
Although they seem axiomatic, these reactions to light and darkness are precisely controlled by a 'master clock' in our brain called the suprachiasmatic nucleus, or body clock. The body clock uses signals like light and darkness to know when to release certain hormones and neurotransmitters that tell us when to wake up or go to sleep. This daily cycle is known as your circadian rhythm (sir-Kadian, is Latin for 'about a day'). Circadian rhythms tell your body when to wake up, how active or energetic to be, and when to pull back and go to sleep.
During the morning, increasing light (usually sunshine) signals the body clock that it should begin its active cycle. The body clock responds by producing serotonin, adrenalin and cortisol, etc. Serotonin is associated with wellness and cerebral activity. Many sleep experts think that increased serotonin causes us to become conscious again after sleep.
Adrenalin helps with motor activity and energy, but it's not enough to pull us out of sleep and wake us up. That role belongs to cortisol. Cortisol could be thought of as jumper cables to get your motor going in the morning. Cortisol also looks and acts very similar to adrenalin, except that higher levels of cortisol are only meant to be in the system for short durations. As morning wears on, the body clock causes cortisol levels drop, while continuing to increase adrenalin and serotonin.
As the level of these hormones rises, so does body temperature and metabolism. The body clock signals us to have an appetite, and it signals the liver, stomach and other organs to process nutrients. This process is evident to anyone who has traveled more than a few time zones. It usually takes a few days to lose hunger pangs caused by the old time zone. By mid afternoon, the body's metabolism has reached its peak; exercise and other physical activity are best performed at this time, as the body is most efficient now at converting fat into energy. High fat or caloric meals are best eaten during midday, as the body is best equipped to metabolize them.
As daylight signals begin to fade, the body clock cuts back on the active, energetic hormones. Within a few hours, body temperature begins to fall and we begin to wind down. As daylight intensity continues to diminish, the body clock signals the pineal gland to convert serotonin into melatonin. The Dim Light Melatonin Onset (DLMO)1 occurs, and we become further lethargic. As the body clock increases melatonin and other sleep hormones, our temperature continues to drop, and we start thinking about withdrawing and going to sleep. As melatonin increases, our bodies slip into a hibernation response and become very efficient at holding on to carbohydrates and fatty foods while converting them into fatty tissue for storage through a long night. For this reason, meals high in fats and carbohydrates should not be consumed more than 4 hours before bedtime.
Approximately 2 hours after the DLMO, melatonin and other nighttime hormones now flood the endocrine system and blood stream. Although melatonin hasn't reached its peak, it is flowing faster now than it will through the rest of the night, and it is more difficult to stay awake or be active. This is the best time to fall asleep; within an hour, the release of melatonin will slow down and it will become more difficult to fall asleep. Body temperature will continue to drop as melatonin is released into the bloodstream.2
Melatonin continues to be released until the body clock perceives a gradual increase of light. As darkness begins to fade into twilight, the body clock shuts down the production of melatonin. This usually occurs an hour or two before awakening, and this point is referred to as the Body Temperature Minimum (BTM). As the body clock is cutting melatonin production, it begins the active cycle again, releasing cortisol. As sunlight increases, the body clock begins producing adrenalin and serotonin.
The body clock is most active when body temperature is at its lowest point, because that is when it is switching from a night to an active cycle. This is also the time when the body clock is most strongly influenced by light and dark signals. If body clock receives light just after the BTM, it will have the most dramatic effect on advancing or speeding up its circadian rhythm, so that you will awaken and fall asleep much earlier. Conversely, if light is received just prior to the BTM, it will have the most pronounced effect at delaying or slowing down circadian rhythms, causing you to awaken and fall asleep later.
Because the body clock intricately controls the sleep wake process, it is no wonder that most sleep problems have something to do with a malfunctioning body clock. Sleep experts recognize the importance rhythms and advocate sleep regimens that keep as consistent schedules as possible. Dr. Michael Smolenski, a leading chronobiologist and author of The Body Clock Guide to Better Health, reports that most sleep disorders involve circadian rhythm disturbances and that 1/4 th of all sleep disorders are categorized as circadian rhythm sleep disorders.
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