Ingrained in all of us is a natural instinct for continuity in our sleeping and waking schedule each day -- our circadian rhythms. While we can choose to ignore this need for continuity by working at night or sleeping late into the day, it carries consequences. The circadian system drives our ability to sleep at night and be alert during the day, so trying to stay awake at adverse circadian times -- like the middle of the night -- may prevent us from being able to stay awake when we need to or fall asleep when we choose.
The circadian system is driven by a master clock in the brain located at the base of the hypothalamus called the suprachiasmatic nucleus (SCN). This small cluster of cells influences bodily processes including sleep-wake, feeding, hormonal secretion and gene expression. The output of these cells combined with the neural processes regulating sleep and wakefulness profoundly influences most human behaviors.
The circadian clock must be reset each day. This is primarily accomplished by exposure to light, which, depending on the time of exposure, can adjust the timing of our circadian clock to synchronize with the light-dark cycle of the planet.
In most of us, the circadian system encourages a schedule compatible with normal business hours -- it begins to promote sleep fairly early in the evening, allowing sleep to begin at about 10 p.m. to midnight in most people, and helps us get out of bed between 6 o'clock and 8 o'clock in the morning. However, this is not always the case as in these examples:
There are also actual circadian rhythm disorders. For example, in some people, commonly adolescents and younger adults, the clock tends to be delayed or run late. Think about this common scenario -- a high school sophomore who can't fall asleep until 1 or 2 o'clock in the morning, but still has to get up at 6:30 a.m. to go to school. Being awake during the morning at school when his or her circadian rhythm wants to sleep can cause sleep deprivation and result in poor academic performance.
This problem is called delayed sleep phase disorder. If someone with this condition tries to go to bed at a normal time, falling asleep will be a problem. Getting up in the morning at a normal hour may be difficult as well. On the other hand, individuals with this disorder rarely have trouble staying asleep once they fall asleep and can easily sleep late into the morning or even the early afternoon if allowed to do so. (Again, think of that high-schooler who stays up to all hours of the night, but sleeps in until 1 p.m. on the weekends.) In my experience, this disorder is best treated with low dose melatonin administered several hours before the desired bed time or bright light in the morning.
The circadian clock can be advanced as well, although this is less common and tends to occur in older individuals. A person afflicted with this disorder would generally get sleepy early in the evening and wake from sleep early in the morning with difficulty returning to sleep. This is called advanced sleep phase disorder and is best treated with bright light in the evening.
Our circadian clock clearly drives both sleep and wake. It is often difficult to sleep when your circadian rhythm is pushing for wakefulness, or equally as challenging to perform optimally if your clock says you should be asleep. There are actions each of us can take to support our circadian rhythms, including going to sleep and waking up at the same time each day; being exposed to bright light during the day and minimizing this exposure during the evening.
As there is not currently a clinically available test to define circadian phase, or body clock time, circadian rhythm disorders are generally diagnosed based on patient history. This approach probably works reasonably well for individuals with extreme circadian problems, but more subtle disorders are likely missed and will continue to be until formal testing methodologies become available.