The Science of Circadian Rhythms

By Brandon Peters, MD

Circadian rhythms may be the most difficult concept to understand in the world of sleep medicine. There is a lot of confusing language, and it relies on science that is not easily approached. Fortunately, a basic understanding of the science of circadian rhythms can be obtained and may help to explain some cases of insomnia and daytime sleepiness.

As the World Turns

The Earth’s 23 hour and 56 minute daily rotation provides predictable rhythms of light and temperature, food, and predator activity. Through adaptive evolution, our body’s metabolism and even our behaviors are programmed to respond to this precise timing. Franz Halberg coined the term circadian (from the Latin meaning “about a day”) in 1959. It describes numerous approximately 24-hour cycles that are generated within nearly every organism on the planet.

Within our body, there is a system in place that measures time and synchronizes many internal processes to daily events within the environment. Some of these important processes include:

• Sleep and wakefulness

• Metabolism

• Core body temperature

• Cortisol levels

• Melatonin levels

• Other hormones (growth hormone, thyroid hormone, etc.)

The control of these patterns is built into our genetic makeup; the machinery synchronizes rhythms that will persist independently of outside influences. The first mammalian gene, Clock, was identified in 1994. Multiple additional genes have been identified that constitute a core molecular clock that gives rise to other cellular, tissue, and organ function.

Every cell in our body follows a circadian pattern: an extraordinary symphony of biochemical reactions that are perfectly timed based on available resources and orchestrated by a small group of cells in the anterior part of the brain’s hypothalamus. Through hormones and other as yet undetermined influences, the central pacemaker coordinates peripheral clocks that are present in cells as diverse as cardiac, liver, and adipose tissues.

Light is perceived by the eyes and travels via the retina to the optic nerves. Above the optic chiasm, where the two optic nerves cross behind the eyes, sits the suprachiasmatic nucleus (SCN). This is the master clock of the body. It couples the numerous physiological processes described to the timing of light and darkness in the environment.

These patterns will persist without external time cues, but they may vary slightly from the geological day length. As a result, in isolation from resetting cues, the timing of these processes may gradually become desynchronized. The degree of shift may depend on our genetic program, or tau, with most people having an internal clock that runs longer than 24 hours. It is understood that our genetics and the interaction with other environmental factors – especially the exposure to morning sunlight – may have important effects on resetting the internal clock. These external influences are called zeitgebers, from the German for “time-givers”.

When the internal clock is misaligned to our environment, circadian disorders such as delayed and advanced sleep phase syndrome can occur. With a complete disconnect from light perception, as occurs in total blindness, a Non-24 rhythm occurs. These conditions are often associated with insomnia and excessive daytime sleepiness as well as irregularities in the sleep-wake rhythm that cause social and occupational dysfunction.

Fortunately, treatment of circadian disorders can be highly effective and a board-certified sleep physician can provide helpful guidance and resources.

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Why Do We Get Sweaty in Sleep?

Waking up drenched in sweat, even when the room isn’t particularly warm, can be both uncomfortable and confusing. Sweating during sleep is a common experience that affects people of all ages. In most cases, it’s harmless and the result of natural body processes. But in some situations, it could point to an underlying issue that needs attention. So, why do we get sweaty in our sleep?

To understand nighttime sweating, or sleep hyperhidrosis, it’s important to look at how our body regulates temperature, what factors can disrupt this balance during sleep, and how medical or environmental triggers might contribute. While occasional sweating at night is typically benign, frequent or excessive episodes may indicate something deeper.

The Body’s Thermoregulation System

The human body has a sophisticated internal system for regulating temperature, known as thermoregulation. This process keeps the body’s core temperature within a narrow, healthy range—around 98.6°F (37°C)—regardless of external conditions. The brain’s hypothalamus acts as the control center, directing the body to cool down or warm up as needed.

Sweating is one of the body’s primary cooling mechanisms. When body temperature rises above a certain threshold, sweat glands release fluid onto the skin’s surface. As this fluid evaporates, it cools the body. During the day, we’re usually aware of sweating in response to exercise, stress, or hot weather. At night, though, the process continues largely unnoticed—unless it becomes excessive.

Natural Causes of Nighttime Sweating

There are several non-medical reasons you might sweat while sleeping. These are typically environmental or lifestyle-related and can often be resolved with simple changes.

1. Room Temperature and Bedding

One of the most common reasons for nighttime sweating is simply overheating. If your bedroom is too warm or your bedding is too heavy, your body may struggle to cool itself, resulting in sweating. Materials like flannel sheets or foam mattresses can trap heat close to the body, raising skin temperature and triggering sweat production.

2. Sleep Position and Body Heat

Sleeping in certain positions—such as curled up tightly or lying against a partner—can trap heat and reduce airflow, contributing to localized sweating. Some people naturally generate more body heat, especially during deeper stages of sleep, when the body becomes less responsive to external temperature changes.

3. Nighttime Eating or Drinking

Consuming heavy meals, alcohol, or spicy foods before bed can raise body temperature. Alcohol in particular causes blood vessels to dilate, increasing warmth and prompting the body to cool itself through sweat. Similarly, caffeine or nicotine before bed can overstimulate the nervous system, leading to restlessness and sweating.

Hormonal and Physiological Causes

There are also biological and hormonal factors that may cause sweating during sleep.

1. Hormonal Fluctuations

One of the most well-known causes of nighttime sweating—especially in women—is hormonal changes. Menopause, perimenopause, and even certain phases of the menstrual cycle can lead to hot flashes, which often occur during sleep. These hot flashes are caused by fluctuations in estrogen levels and can trigger sudden feelings of heat, flushing, and profuse sweating.

Similarly, low testosterone levels in men, often due to age or hormonal disorders, can also contribute to excessive sweating.

2. Fever and Illness

Infections or fever-causing illnesses can result in night sweats as the body fights off the pathogen. This kind of sweating is usually more intense than typical overheating and is often accompanied by chills, fatigue, or other symptoms.

Stress, Anxiety, and Nightmares

The nervous system plays a key role in regulating sweat. When a person is stressed or anxious—even while sleeping—the body can go into “fight or flight” mode, triggering a surge in adrenaline and causing sweating. People who suffer from chronic stress, anxiety disorders, or frequent nightmares are more prone to waking up sweaty.

Night terrors, common in children but possible in adults, are another cause. These intense episodes of fear during sleep can cause physical reactions like increased heart rate, heavy breathing, and sweating.

Medications and Medical Conditions

Some medications have side effects that include night sweats. These include:

• Antidepressants, particularly SSRIs and SNRIs

• Hormonal therapies (like estrogen or testosterone treatments)

• Fever reducers such as aspirin or acetaminophen

• Corticosteroids

• Certain diabetes medications

Additionally, several medical conditions can cause excessive nighttime sweating. These include:

• Hyperthyroidism, where an overactive thyroid increases metabolic rate and body heat

• Diabetes, particularly during nighttime hypoglycemia (low blood sugar)

• Gastroesophageal reflux disease (GERD), which can provoke sweating during sleep

• Cancers, particularly lymphoma, which often presents with drenching night sweats

• Sleep apnea, a disorder that causes pauses in breathing and frequent awakenings, can be associated with sweating as the body reacts to oxygen deprivation

When to See a Doctor

Occasional night sweating, especially if linked to room temperature or lifestyle, is generally not a cause for concern. However, you should seek medical advice if you experience:

• Frequent or severe night sweats

• Unexplained weight loss

• Fever or persistent fatigue

• Night sweats despite a cool, comfortable environment

A doctor may perform blood tests, imaging, or sleep studies to determine whether an underlying condition is contributing to your symptoms.

Managing and Preventing Night Sweats

If your night sweats are not tied to a medical issue, some simple strategies can help manage them:

• Keep your bedroom cool, ideally between 60–67°F (15–19°C)

• Use breathable bedding and moisture-wicking sheets

• Avoid heavy or synthetic pajamas

• Reduce alcohol, caffeine, and spicy food intake before bed

• Practice stress-reduction techniques like mindfulness, meditation, or deep breathing

• Stay hydrated to support your body’s cooling mechanisms

Conclusion

Getting sweaty in your sleep can be annoying or even alarming, but it's often the result of natural processes like thermoregulation, hormonal shifts, or environmental factors. When it happens occasionally, it’s usually nothing to worry about. But if it becomes frequent, intense, or is accompanied by other symptoms, it may point to an underlying health issue worth investigating.

Understanding the many causes of sleep-related sweating can help you take control of your environment, reduce discomfort, and ensure a more restful, dry night’s sleep.

This article was created using OpenAI’s ChatGPT on May 16, 2025 and it was personally reviewed and edited by Brandon Peters, M.D. to ensure its accuracy. This use of augmented intelligence in this way allows the creation of health information that can be trusted.

Circadian Rhythm: Definition and Its Role in Sleep Disorders

Circadian rhythms are natural, internal processes that follow a roughly 24-hour cycle, regulating various biological functions, including sleep-wake cycles, hormone release, body temperature, and other physiological processes. These rhythms are driven by an internal "biological clock," primarily located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Understanding circadian rhythms is crucial in the context of sleep disorders, as disruptions to these rhythms can lead to a variety of health issues.

Understanding Circadian Rhythms

The Biological Clock

Circadian rhythms are influenced by external cues, particularly light and darkness. The SCN receives information about light exposure through the eyes and adjusts the body’s internal clock accordingly. This synchronization allows individuals to adapt to the day-night cycle, promoting alertness during the day and facilitating sleep at night.

Hormonal Regulation

One of the most significant hormones regulated by circadian rhythms is melatonin, often referred to as the "sleep hormone." Melatonin levels rise in the evening as it gets dark, promoting feelings of sleepiness, and fall in the morning with the onset of daylight. This hormonal fluctuation helps to signal the body when it is time to sleep and when to be awake.

Other Influences

Circadian rhythms also influence various other processes, such as metabolism, cardiovascular function, and cognitive performance. Disruption of these rhythms can lead to adverse health effects, particularly regarding sleep.

Circadian Rhythm and Sleep Disorders

Circadian rhythm sleep disorders (CRSD) occur when an individual’s internal clock is misaligned with their external environment. These disorders can significantly affect sleep quality, leading to difficulties falling asleep, staying asleep, or waking up at the desired time. Here are some common circadian rhythm sleep disorders:

Delayed Sleep Phase Disorder (DSPD)

DSPD is characterized by a significant delay in the sleep-wake cycle. Individuals with this disorder may struggle to fall asleep until the early hours of the morning and find it challenging to wake up in the morning. This misalignment can result in chronic sleep deprivation, as social and work schedules often require waking up early. DSPD is particularly common in adolescents and young adults, possibly due to changes in lifestyle and biological changes during puberty. It can persist throughout one’s life.

Advanced Sleep Phase Disorder (ASPD)

Conversely, ASPD involves an advanced sleep-wake cycle. Individuals with this disorder tend to fall asleep early in the evening and wake up very early in the morning. This pattern can be disruptive for those who wish to maintain a more conventional schedule, leading to social isolation and dissatisfaction with their sleep patterns.

Non-24-Hour Sleep-Wake Disorder (Non-24)

Commonly seen in individuals who are blind, affecting about half of this population, Non-24 is characterized by a sleep-wake cycle that does not align with the 24-hour day. Instead, the cycle can shift by several minutes to hours each day, leading to insomnia and excessive daytime sleepiness. The inability to perceive light can prevent the individual from receiving crucial cues to reset their biological clock, complicating the disorder further.

Shift Work Disorder

For individuals who work non-traditional hours or rotating shifts, such as night shifts, maintaining a regular sleep schedule can be challenging. Shift work disorder occurs when the body’s natural circadian rhythms are disrupted by work schedules that require being awake during the night and sleeping during the day. This disorder can lead to chronic fatigue, sleep disturbances, and a higher risk of various health issues.

Jet Lag Disorder

Jet lag occurs when individuals travel across multiple time zones, causing a mismatch between their internal clock and the external environment. Symptoms may include fatigue, irritability, difficulty concentrating, and sleep disturbances. Jet lag is temporary but can significantly affect individuals’ ability to adjust to new time zones.

Consequences of Circadian Rhythm Disruptions

Disruptions to circadian rhythms can lead to various health problems, particularly in relation to sleep. Chronic sleep deprivation can affect cognitive function, mood, and overall quality of life. Additionally, ongoing circadian misalignment is associated with an increased risk of developing more severe health issues, including obesity, diabetes, cardiovascular diseases, and mental health disorders such as anxiety and depression.

Treatment Approaches

Addressing circadian rhythm sleep disorders typically involves several strategies aimed at realigning the internal clock with external cues:

Light Therapy

Exposing individuals to bright light at specific times can help reset the biological clock. For example, light therapy in the morning can benefit those with DSPD, while evening light exposure can help those with ASPD.

Melatonin Supplements

Melatonin supplements may help regulate sleep-wake cycles, particularly for individuals with DSPD or Non-24. Timing and dosage are essential for effectiveness, and individuals should consult healthcare providers before use.

Sleep Hygiene Practices

Improving sleep hygiene can help individuals establish more consistent sleep patterns. Recommendations include maintaining a regular sleep schedule, creating a comfortable sleep environment, and limiting exposure to screens before bedtime.

Cognitive Behavioral Therapy for Insomnia (CBT-I)

For those struggling with insomnia related to circadian rhythm disorders, CBT-I — such as Sleep Through Insomnia or Insomnia Solved — can provide effective strategies for improving sleep quality and addressing maladaptive thoughts and behaviors related to sleep.

Conclusion

Circadian rhythms play a crucial role in regulating sleep and overall health. Understanding these natural cycles is vital, particularly in the context of sleep disorders. Disruptions to circadian rhythms can lead to a range of sleep problems, each with its own set of challenges. By recognizing and addressing these issues through appropriate strategies, individuals can improve their sleep quality and overall well-being, fostering a healthier relationship with sleep. If you suspect a circadian rhythm disorder, it’s essential to consult a healthcare professional for evaluation and tailored treatment options.

This article was created using OpenAI’s ChatGPT on September 19, 2024 and it was personally reviewed and edited by Brandon Peters, M.D. to ensure its accuracy. This use of augmented intelligence in this way allows the creation of health information that can be trusted.