Sleep Disorders

Circadian Cycles and Sleep

Here’s an overly simplistic explanation of sleep: it’s part of the circadian rythym of life and is hardwired into the biology of persons and animals. Your brain wants to sleep when it gets dark and wake when it is light.

That’s partly true, but it’s not the whole story. A more precise explanation is that the sleep cycle stems from an interaction between the circadian clock and a separate sleep-wake homeostatic process. The "sleep homeostat" is, roughly, an accounting of the amount of sleep you’ve experienced recently. It causes the sleep drive to be based on how much sleep you’ve got in the past, and is directly related to the concept of sleep debt or sleep deficit. The sleep homeostat is similar to the hunger homoestat. If you haven’t eaten in a while, you’re likely to be hungry regardless of the time of day. If you had a feast at lunch, you may not be hungry come dinnertime. Likewise, if you’ve stayed awake all night, you’ll probably feel like sleeping in the morning, even if the Sun is up.

However, that’s not to say that the circadian cycle doesn’t matter. Cues such as daylight and regularly scheduled social and family activity have powerful influences on how sleepy or awake a person feels. These cues affect the internal clock. Disturbances of the normal circadian rhythmicity can result in serious health consequences, including psychiatric disorders, such as depression.

Blind people often experience sleeping problems because their retinas are unable to detect light and they don’t have the circadian cues of daylight and night. Shift workers try to run their lives out of sync with light and dark cycles and consequently have problems.

To reduce the effects of jet lag, some therapists try to manipulate the biological clock with a technique called light therapy. They expose people to special lights, many times brighter than ordinary household light, for several hours near the time the subjects want to wake up. This helps them reset their biological clocks and adjust to a new time zone.

Disturbed circadian rhythms have been associated with a variety of mental and physical disorders and may negatively impact safety, performance, and productivity. Many adverse effects of disrupted circadian rhythmicity may be linked to disturbances in the sleep-wake cycle. Some rhythmic processes are more affected by the circadian clock than by the sleep-wake state, whereas other rhythms are more dependent on the sleep-wake state.

Circadian rhythms are ubiquitous in the animal kingdom, and are a cellular property. Neurons in a dish can act as clocks. The genes responsible for this cyclic behavior have begun to be identified. Clocks enable organisms to adapt to their surroundings. Although scientists currently believe that clocks arose through independent evolution and may use different clock proteins, they all share several regulatory characteristics. In particular, they are maintained by a biochemical process known as a negative feedback loop. Another hormonal cycle related to sleep is a reciprocal interaction of the neuropeptides growth hormone-releasing hormone and corticotropin-releasing hormone.

Dissecting the mechanism of our internal clock - from the National Center for Biotechnology Information

Circadan Rythyms - Cycles of Nature

Light-dark cycle synchronization of circadian rhythm in blind primates