Many of us have been there before: waking in the middle of the night, often in response to a dream or nightmare, afraid and covered in perspiration. 

While sweating has many benefits, such as being a "natural function that the body uses to regulate body temperature," explains Jewel Kling, MD, assistant director of women's health for Mayo Clinic, it can also be a source of concern. 

For instance, some people worry that their sweat causes body odor, while others fret that their obvious sweating is a sign they're nervous or anxious (which it may well be, but they don't want others to know that). But for people who sweat a lot at night, perspiration can be especially disruptive to a healthy sleep schedule, and even costly due to the amount of laundry it can create. 

What are night sweats? 

Nights sweats are repeated episodes of very excessive sweating that occur during sleep - often heavy enough to soak through nightclothes or bedding, per Mayo Clinic. Such episodes can occur whether one's sleeping environment is hot or cold. 

"Night sweats can range from moderate to excessive with drenching sweats," explains Kling. Like episodes of excessive sweating that can occur during the day, night sweats are usually considered generalized hyperhidrosis, or heavy sweating. Such perspiration can happen on its own or it may be accompanied by an illness such as fever or nausea. 

What causes night sweats? 

While various things can cause or contribute to night sweats including medication and medical conditions, "it is recommended that when you frequently experience night sweat, you have it evaluated by your clinician to identify the cause," suggests Kling. 

Another such cause many include hormonal changes like menopause, which can make it hard for the brain to regulate body temperature, often causing night sweat. Infection can contribute as well; as can night terrors or bad dreams. "Stress, increased heart rate, and other stimuli can trigger night sweating," says Anthony Beutler, MD, an associate medical director of sports medicine at Intermountain Health in Salt Lake City.

Other culprits of night sweats include vitamin B12 deficiency, excess weight, dysfunction within the autonomic nervous system or certain disorders of the endocrine system such as thyroid disease. 

And each such factor can lead to different degrees of night sweating. "The composition of sweat varies considerably, depending on its cause," explains Loren Fishman, MD, a professor of physical medicine and rehabilitation at Columbia University.  

How are night sweats different than exercise-related sweat?

It's also worth noting that night sweat is not the same as exercise-related sweat in a couple distinct ways. "The chemical composition of perspiration from night sweats, like that from other emotional or endocrine causes, is different in its chemical composition from exercise-induced perspiration," says Fishman. To explain this, he says that sweat from normal activity is higher in lactate - a product of metabolism - than the sweat produced while sleeping.

Kling adds that night sweat and exercise-related sweat also have different functions, even if the mechanisms that lead to perspiration are similar. "Exercise-related sweating is triggered by the body during exercise to control body temperature," she explains. "Night sweats are typically caused by something else." 

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