The human body follows an internal timekeeping system known as a circadian clock. This internal clock regulates the body’s natural “circadian rhythm,” your daily cycles of sleep and wakefulness, hunger and digestion, hormonal activity, and other bodily processes. The word circadian comes from the Latin phrase “circa diem”, meaning “about a day”, referring to how most circadian rhythms automatically reset every 24 hours. Circadian rhythms are guided by natural signs that you should be awake like light exposure, interaction with people, and planned meal times. However, once set, circadian rhythms can be quite difficult to change, preserving the rhythm without any exposure to the typical signals.
The circadian clock consists of a cluster of roughly 20,000 neurons known as the suprachiasmatic nucleus (SCN). The cluster is located in the hypothalamus at the base of the brain. When the eyes perceive light during the day, they activate signals that travel down a nerve tract to the SCN, which lets the brain know it is time to be awake. The SCN then releases a series of hormones, including cortisol, making sure you are awake and perky for your 9:00 am meeting.
The body uses light and other signals, called ‘zeitgebers’ (German for “time giver” or “synchronizer”) to determine whether it is day or night and to synchronize circadian rhythms accordingly. Light is considered the most important zeitgeber for the circadian rhythm. Even when our eyes are closed, the eyes still perceive light and activate signals to the SCN. Other zeitgebers include physical activity, food intake, body temperature, and social interaction.
Circadian rhythms regulate the production of different hormones throughout the 24-hour cycle. When the sun rises in the morning, the body produces cortisol, a hormone that makes us feel refreshed and alert. After waking, a healthy person will become increasingly tired throughout the day until the sun goes down, when feelings of tiredness peak. As the sun begins to set, the pineal gland will release melatonin, a hormone that reduces wakefulness and alertness.
Circadian rhythms also regulate hunger and digestion, body temperature, mood, fluid balance, and other important physiological processes. For most healthy adults, the circadian clock will reset every 24 hours. However, there are variations on when people feel tired and when they feel alert throughout the day. Two examples are “early risers,” who go to bed and wake up early, and “night owls” who go to bed relatively late and then sleep in.
Your sleep rhythm will also evolve and change with age. For example, older people tend to go to sleep and wake up earlier in the day than younger people, while babies will sleep in multiple phases throughout the day and night.
Genes or late nights working the graveyard shift may incline your circadian rhythm one way or another. Some methods of changing your sleep schedule are less effective than others. Consuming alcohol is one example of a less effective strategy. Alcohol is a central nervous system depressant that induces feelings of sleepiness after it is consumed, so many people drink in order to feel more tired and relaxed before bed. However, alcohol also lowers sleep quality and duration, making sleep un-restorative and choppy.
Sleep medications are also questionable as long-term strategies. When prescribed properly, some medications can help you acclimate to a new sleep schedule or get you through a significantly stressful period that is affecting your sleep. However, sleeping pills are a temporary fix and they won’t alter your circadian clock. Additionally, some sleep medications can leave you feeling drowsy when you wake up.
In order to effectively alter your circadian rhythm and sleep-wake cycle, we recommend the following techniques:
If you’d like to change your sleep schedule, consult with your doctor or another credentialed physician about the safest and healthiest measures that are right for your particular circadian goals.
The master circadian clock in healthy adults will operate on a daily cycle that resets about every 24 hours. Circadian rhythm sleep disorders are caused by delays, advances, and complete dysregulation of a person’s circadian cycle. These disorders can take different forms – though, for most, disturbed sleep and excessive daytime sleepiness are common symptoms.
Some can be attributed to a timing issue in a person’s internal timekeeping system. For instance, delayed or advanced sleep-wake phase disorder occurs when someone’s sleep-wake cycle falls at least two hours later or earlier than traditional circadian schedules. Another example is irregular sleep-wake rhythm disorder, which is characterized by fragmented sleep-wake patterns that produce disturbed sleep when the person tries to sleep and grogginess when they are awake. This disorder is often seen in people with Alzheimer’s disease, Parkinson’s disease, and other neurodegenerative conditions.
Other circadian rhythm disorders are due to misalignments between someone’s circadian clock and their external environment. Shift work disorder, a common condition among people who work late at night or early in the morning, can cause excessive daytime sleepiness and also make it difficult for them to fall asleep at their scheduled bedtime. Another example is jet lag, a condition that affects people who travel across multiple time zones during a relatively short period of time. Jet lag temporarily causes fatigue and disrupts sleep as the traveler’s body adjusts to the new local time.
Most circadian rhythm sleep disorders are diagnosed after patients exhibit symptoms for at least three months. While each condition above requires a specific diagnosis, many can be treated by sleep specialists with measures we’ve described above, such as light exposure therapy and melatonin supplements. Improving sleep hygiene and following a consistent bedtime schedule may also be effective. If left untreated, these disorders can have negative impacts on your physical, cognitive, professional, and social performance.