Circadian Rhythm defects can come many distinct forms depending on the amount and timing of waking up and sleeping. Others, as in the case of Seasonal Affective Disorder (SAD), result in the changes in the length of day and night.
People who tend to sleep at a later time of the day and wake up late as well are said to be exhibiting Delayed Sleep Phase Syndrome (DSPS). These people tend to develop insomnia, an abnormal inability to take adequate amount of sleep due to not being able to sleep at the right time of the night.
Still, other people, especially the elderly, are more likely to sleep early around 7:00 PM and consequently wake up early around 1:00 AM or 2:00 AM. This is a symptom of disrupted natural biological rhythms known as Familial Advanced Sleep-Phase Syndrome (FASPS).
The amount of a substance called melatonin is also perceived as a motivating factor in sleep-related disorders in humans. In a study among vertebrate mammals, melatonin is secreted in response to the absence of light.
This means more melatonin is secreted in the period of darkness and less in the presence of light. It presupposes the idea that melatonin is related to the 24-hour sleep/wake cycle of every human being.