Circadian rhythm sleep disorders are disorders of the timing of sleep and wakefulness. We all have an internal "clock." This "clock" controls our rhythm of sleeping and wakefulness.
There are times of the day when we are most alert. This is often mid-morning and early evening. And, there are times when we are more likely to be sleepy - during early to mid-afternoon and late evening.
A person's environment impacts the timing of sleep and wakefulness, and may either complement or disrupt our internal clock.
Excessive sleepiness can develop when our sleep rhythm is altered. We may want to sleep during times when we need to remain awake. On the other hand, insomnia occurs when we attempt to sleep at a time when our rhythm is to stay awake.