This interesting group of disorders involves the body’s internal clock not being in harmony or synchrony with the external light/dark cycle or the social cycle. The hallmark of these disorders is that the same patient complains of sleepiness during one part of the day and then insomnia during another part of the day.
There are two basic types of circadian rhythm disorders: internally driven and externally driven. The internal types include a disorder common in teenagers: Delayed Sleep Phase Disorder (DSPD). In this illness, the internal clock of these teenagers shifts several hours into the night, such that instead of going to sleep at 10pm they now need to begin sleep at 1 or 2am. Thus, they tend to wake up late. During school days they become sleep deprived, with all the attendant complications such as doing poorly in their early morning classes.
A similar internal type found in the elderly (but in the opposite time period) is Advanced Sleep Phase Disorder (ASPD): in which the person needs to go to sleep earlier in the evening, usually around 8pm, but then wakes up at 4am. Thus, the patient complains of sleepiness in the evening, which disrupts his social schedule, but then complains of insomnia in the early morning. Sleeping pills do not work in this situation.
The main externally driven disorder is Shift Work Syndrome that affects more than 10% of nighttime workers. Unfortunately, the patient becomes sleepy and tired during work hours and job performance is affected. At the same time, this patient suffers from insomnia when he or she does want to sleep. It is the worst of both worlds. Even though this very common disorder has plagued nighttime workers for centuries, it has only begun to be studied extensively within the last 20 years. Thankfully, there are various management techniques and medications available to control this disorder.
Jet lag is another externally driven disorder that is common in this age of business globalization and world travel. If only a few time zones are traversed (particularly if one travels east), a little patience is usually all that's required for the body to naturally return to its correct circadian rhythm. At other times, medications may be necessary.
An interesting aspect of the treatment of circadian rhythm disorders is the use of light therapy, since light so deeply influences the body's internal clock.