It’s a known fact that sleep deprivation can precipitate mania in people with bipolar disorder. Psychology Today reports that even one night of total sleep deprivation can destabilize mood to the point where hospitalization is needed.

To understand the relationship between sleep and mood, you need to understand a little about chronobiology (from khronos, the Greek word for time), the science of bodily rhythms and biological clocks. Nearly every living thing has an internal biological clock; even organisms as simple as bacteria have an internal time-keeping mechanism. Humans are no different.

An important concept in chronobiology is that of circadian rhythms. The term comes from the Latin words circa, meaning “around,” and diem, meaning “day.” It refers to rhythms in the body that have an approximately 24-hour cycle, like the day-night cycle. Many bodily functions follow a circadian rhythm: body temperature cycles daily as does blood pressure. It has been demonstrated experimentally that our biological clock has a natural cycle of about 25 hours and needs to be reset daily by environmental cues called zeitgebers, a German word meaning “time-givers,” that tweak our internal clock every morning to keep us on a 24-hour schedule. Although our cycle can respond to these daily tweaks without any trouble, this “reset” can shift by only about one or two hours at a time. If the zeitgeber suddenly starts arriving several hours off schedule, as happens with jet lag, it takes several days for the cycle to catch up.

During that time, we awaken and get sleepy at the wrong times and generally feel out of sorts. Eventually, our internal clock adjusts, but the farther we’ve traveled, the longer it takes: about one day for each hour of time change. What happens to people whose internal clocks are out of sync with their environment for more prolonged periods?

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