Our natural sleep wake circadian rhythm does not usually last exactly 24 hours. Some people have an earlier sleep phase with a circadian rhythm that lasts less than 24 hours, whereas some people have a later sleep phase with a circadian rhythm that lasts longer than 24 hours. People with a shorter than 24-hour sleep phase are the typical morning larks that jump out of bed in the morning with boundless energy and people who can stay up all night partying are the night owls.
Re rely on zeitgebers (environmental time cues) such as the circadian day night cycle to entrain our sleep wake cycle to the length of a standard day, i.e., 24 hours. Nonetheless some people experience difficulties with such entrainment, and problems can occur, usually at the extremes of age.
Teenagers have difficulty getting up in the morning, but enjoy staying up late at night, and they can be considered to have delayed sleep phase disorder, whereas it is common for elderly patients to wake up too early in the morning and they can be considered as having advanced sleep phase disorder.
The administration of melatonin in the evening, in conjunction with early morning light exposure can help people with delayed sleep phase disorder. The administration of melatonin in the morning in conjunction with morning darkness can help people with advanced sleep phase disorder.
Jet lag is also a form of sleep phase disorder. It usually occurs when we cross five or more time zones. When we travel eastward, we chase the sunrise and experience delayed sleep phase disorder, i.e., we feel awake when everyone else is trying to get to sleep. Light exposure in the early part of the day at the destination and melatonin in the destination's evening will help. When we travel westward, we chase the sunset and experience advanced sleep phase disorder, i.e., we want to go to bed when everyone else is still awake. Exposure to bright light in the late afternoon and evening will help us stay awake until a more reasonable bedtime