A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Do I sweat a liter every night?

January 14, 2011

Dear Cecil:

On the radio, a spruiker from a mattress company claimed it was "common knowledge" that a human sweats a liter of water every night, and for health reasons you should buy a new mattress every couple of years. Can this be true?

Cecil replies:

Wonderful word, spruiker (rhymes with "spook 'er")— slang in Australia and New Zealand for a tout or pitchman. In fairness, the idea that you sweat a liter or some other large amount per night isn't limited to Down Under, but rather is common among mattress floggers all over the world — I found one huckster claiming you sweat two to five gallons a night. (This same goof repeated the notion that a mattress is packed full of dust mites, which we've previously debunked.) One doesn't want to get carried away, but let's look at this in a positive light. Is there any scenario under which the claim about a liter of sweat could possibly be true?

1. Normal sleep. No luck here. Sweating while at rest is minimal below 85 degrees. You do lose a certain amount of moisture in your breath and by evaporation through your skin; the latter process, which doesn't involve the sweat glands, is called insensible perspiration. Total water loss by both routes for an average healthy young male averages about 25 milliliters per hour, or 200 milliliters per eight hours of sleep, and much of that is simply exhaled, not absorbed by the mattress.

2. Energetic bed use. Sweat production kicks in once you start exercising. I found a chart from the Australian Institute of Sport giving typical sweat rates for activities ranging from cricket (0.5 liters per hour) to rugby (as much as 2.6 liters per hour). You may say strenuous exertion is incompatible with sleep. Piffle — I've seen the Chicago Bears’ offensive line play an entire game while unconscious. More broadly, I know of a nocturnal activity or two that typically kicks up your perspiration output. But a liter's worth? Every night? Ain't seeing it. We'll have to give this one up too.

3. Sleeping in a warm room. Once the temperature rises above 85, you start sweating even if you're at rest. Tests conducted in the Sonoran Desert found that subjects sitting naked in the shade in 95-degree heat produced 220 milliliters of sweat per hour. Assuming comparable conditions were to prevail at night, you'd lose close to two liters over an eight-hour stretch. But then you wouldn't even be thinking about a new mattress — you'd be shopping for an air conditioner.

4. Night sweats, also known as sleep hyperhidrosis. This one seems more likely. Night sweats are fairly common and can be triggered by lots of things, among them menopause; taking antidepressants or other meds; panic attacks; obesity; low blood sugar episodes in diabetics; eating spicy food; cancer, including Hodgkins and non-Hodgkins lymphoma; and other diseases such as tuberculosis and HIV. Or sometimes by nothing at all — they just happen. I couldn't find any solid numbers on how much moisture is produced, but in extreme cases the sufferer’s bedclothes are often described as being drenched in sweat, and I'm prepared to believe you could lose a liter. So let's give the spruikers the benefit of the doubt and say it's possible for you to sweat a liter a night under certain circumstances. We’ll relegate to the fine print the clarification that those circumstances may include your being panicked, obese, or seriously ill.

Helping to make this a better world

Your article "Does using a gasoline-powered lawn mower produce as much pollution as driving an SUV 300 miles?" [November 12] came to my attention. We want to correct the inconsistencies you found on EPA's website, but in order to do so, I need to know where those numbers appeared. Would you mind sending me the URLs for the pages and documents where you found that information so that we can follow up? Thanks so much for your help!

Cecil replies:

Pleasure to be of service, Jennifer. I've forwarded the requested information. Anything else the federal government needs straightened out — that pesky deficit, say — you let me know.

Questions we're still thinking about

I was just reading your book Triumph of the Straight Dope and I ran across the stuff about equestrian statues and the raised-foot code [horse with one foot raised means rider was wounded in battle; two feet raised means killed; etc]. It got me to wondering — how come you never see a horse statue with all four feet off the ground?

Cecil replies:

Very funny, Victor. However, I disagree that you never see a horse statue with all four feet off the ground. Next time you‘re on a merry-go-round, look down.

Related Posts with Thumbnails


Adair, Eleanor R. and Black, David. "IEEE White Paper - Thermoregulatory Responses" Public Draft, July, 2002.

American Society of Heating, Refrigerating, and Air Conditioning Engineers, Inc., ANSI/ASHRAE 55-1992, "Thermal Environmental Conditions for Human Occupancy," 1992.

Fisher, R.I. et al. "Factors predicting long-term survival in diffuse mixed, histiocytic, or undifferentiated lymphoma" Blood 58 (1981): 45-51.

Mold, James W. et al. "Prevalence of night sweats in primary care patients" The Journal of Family Practice 51.5 (2002): 452-456.

Reed, Susan D. et al. "Night sweats, sleep disturbance, and depression associated with diminished libido in late menopausal transition and early postmenopause: Baseline data from the Herbal Alternatives for Menopause Trial (HALT)"American Journal of Obstetrical Gynecology 196.6 (2007).

Wenger, C. Bruce. Medical Aspects of Harsh Environments, Volume 1 Washington, DC: Office of the Surgeon General, Borden Institute, 2001.

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