If you often wake up in the middle of the night, drenched in sweat – even though it’s cool in your bedroom – you have nocturnal hyperhidrosis, aka the night sweats.
It might just be because you’ve cranked up the central heating, wearing thick pyjamas or it could indicate something far more serious.
On average, we perspire as much as one litre of water every day to help us maintain our constant body temperature of 37°C. This increases at night when we’re wrapped up more warmly in winter or suffering from excessive sweating.
“The body’s temperature control system is very efficient,” says GP Dr Martin Godfrey. “The body has to maintain a constant core temperature, so it has lots of ways of losing heat if it’s too hot, or retaining heat if it’s too cold.
“The system of vasodilation (widening of the blood cells to increase blood flow, which makes your skin flush) is the body’s way of saying we need to lose heat. Your body’s very good at it, but it can feel very uncomfortable.”
Overheating – and sweating – can cause us to wake up in the deepest sleep stage, depriving us of a good night’s sleep and making us more irritable during the day.
“It can be debilitating for people who get it on a regular basis,” says Dr Godfrey.
“Just a few nights of not sleeping makes you tired and irritable – and makes it difficult to work or do what you need to.”
The night sweats are not only uncomfortable for you – they can be unpleasant for anyone you’re sharing a bed with, too.
But if you suffer from them, you’re not alone. “Night sweats are common and will affect different types of people in different ways,” says Dr Godfrey. Here are some of the main causes:
“As a GP, if somebody’s talking about night sweats, you want to make sure there’s nothing serious going on – and lymphoma is one of the causes of night sweats that you want to be certain there isn’t a risk of.”
Night sweats can be an early indication of this type of cancer, which develops in the lymphatic system and causes lymph nodes to swell.
“If there are no [swollen] lymph nodes and nothing making you think something sinister is going on, then if the patient’s a woman and she’s of menopausal age, it could be an early sign of that happening,” explains Dr Godfrey.
Excessive sweating, combined with a raised temperature, can be a sign of infection. According to NHS choices, the most common infection associated with night sweats is tuberculosis, but it can also be a sign of endocarditis (inflammation of the heart valves), osteomyelitis (a bone infection), abscesses and HIV/AIDS.
Dr Godfrey says: “Alcohol can affect people in different ways, but people who have been drinking heavily don’t realise that a lot of vasodilation goes on. So when they’re in bed, they’ll be hot and sweaty, even though they won’t have a temperature.”
5. Change of season
Dr Godfrey says: “In the spring, people often still dress up warmly and have covers on their beds commensurate with it being very cold – and so they then become overheated.”
Sweating at night can also be a side-effect of certain types of medication, including anti-depressants.
So how to cope with the night sweats? Apart from the obvious techniques to cool down – including opening a window and kicking off your bedclothes – Dr Godfrey recommends having a fan in your bedroom. “Cool air is very effective at getting rid of the discomfort and bringing the temperature down,” he says.
“If you’re worried, go and see your doctor,” he advises. “They will probably give you a blood test, looking at your white cells for signs of lymphoma – and if you’re a woman, they may well give you a test to see what your hormone levels are.”