A summer of heatwaves has made everyone aware how hard to sleep it is when it’s hot and humid. We tend to fall asleep later, rise earlier, and sleep for less time in high temperatures. But it’s not only the outside temperature that can interfere with sleep. For many women, waking up with night sweats can be the most difficult aspect of the menopause. So why is being hot at night such a problem for our sleep, and what can we do about it?
Why does being hot interfere with sleep?
Our sleep and body temperature are closely related. Core body temperature naturally follows a regular pattern every 24 hours, changing by about 1C.
We get gradually warmer after waking up, with a steep increase in the late afternoon, when we feel most alert. As it gets dark, our body temperature falls. The steepest rate of cooling happens at the same time as an increase in the hormone melatonin, and these signals cue the body that it’s time to fall asleep. Our temperature continues to fall naturally until about 4 or 5am then starts to rise in preparation for waking, as melatonin levels fall.
Fig. 1. The relationship between the circadian rhythms of core body temperature (solid line) and melatonin (dotted line) for a good sleeper who normally sleeps between 11:00 PM and 7:00 AM. Adapted from Lovato (2013).
We usually cool down at night by sending more blood flow to the skin. As warm blood passes close to the thin blood vessels at the body’s surface, heat is transferred out to the colder air around us, and we cool down.
If our environment gets too hot, above about 25C, this interferes with cooling from the skin. Studies have found that being in a warmer environment interferes with both falling asleep and staying asleep. We tend to have less of the deep, slow wave sleep, which is very physically restorative. We also miss out on REM or rapid eye movement sleep, which is important for keeping us feeling balanced emotionally.
What usually causes sweating?
Our body is constantly working to keep our organs at a safe temperature. We can manage small fluctuations by simply changing blood flow out to the extremities, to cool down, or back in towards our core, to warm up.
If temperature varies by more than a few degrees, the brain brings more active heat regulation strategies into play. If we get too cold, we start to shiver. The contraction of muscles releases heat energy. If we get too hot - for example through vigorous physical activity - our sweat glands release sweat, which is a combination of water and salts. The energy needed for beads of sweat to evaporate from the surface of our skin releases heat energy, and we cool down faster.
Interestingly, we can also sweat when we’re scared, nervous, or stressed. The body’s ‘fight or flight’ stress response, which is designed to help us evade threats, also involves a sweat reflex. Stress-related sweat glands are concentrated in the armpits and groin, and tend to produce a thicker, fattier sweat than heat sweat.
Some people also find themselves sweating after a spicy meal. Capsaicin, the active ingredient in chilli, can fool the temperature sensitive nerves in the mouth into thinking that it’s hot, triggering a sweat response.
Why can the menopause lead to night sweats?
During the menopause, falling and fluctuating levels of oestrogen and progesterone may alter the sensitivity of the hypothalamus - our temperature regulating centre in the brain. It’s like having a faulty thermostat. It means that we become more likely to switch on a vigorous cooling and sweat reflex, even in response to minor rises in temperature, or stressful situations.
This vigorous cooling effort is experienced as intense heat in the chest as the blood flow increases, spreading to the neck and face. Some women can experience dripping perspiration, while others may hardly sweat at all. During the day these episodes are known as hot flushes, and at night, night sweats. They typically last 1 to 5 minutes, but can repeat 20 or 30 times per day.
We don’t yet know exactly why changing hormone levels impact on temperature regulation, but an overactive stress response, hypersensitivity of skin, or changes to metabolism may all contribute.
There are a wide range of other possible causes of night sweats, including infections, stress, alcohol, excess caffeine, low blood sugar and some cancers. Many women experience night sweats at certain times of their menstrual cycle, or during pregnancy. Certain medications, including some antidepressants, steroids and painkillers can have night sweats as a side effect. Chemotherapy and radiation therapy can also cause night sweats. There is also a harmless condition called hyperhidrosis that makes you sweat too much all the time.
So how can you avoid being too hot to sleep?
Experts from the European Insomnia Network (Altena 2022) agree..
Drinking plenty of water during the day can help to improve temperature regulation, day and night.
Aim to keep your bedroom cooler than normal room temperature, around 19C. In summer, keeping the curtains drawn during the day, and keeping a window open at night could help if it’s not too noisy. Air conditioning might be helpful in extreme circumstances, but electric fans are a lot more energy efficient. Avoid the bedroom being cooler than 17C.
Before going to bed, try a cool or lukewarm shower or footbath (not extremely cold or excessively hot).
Choose light cotton pyjamas and bedding over less breathable materials. Pillows and mattresses designed with thermoregulating properties can be helpful.
If you’re sleeping with a partner, try separate covers. In warm weather, if you’re worried about night sweats, you might find that separate beds are better for your sleep.
Alcohol can dehydrate you, and increases nocturnal sweating. Reducing or avoiding alcohol entirely can improve your sleep quality.
Many of the tips which are true for sleep quality in general remain true if you’re getting too hot at night..
If you have a bad night’s sleep one night, don’t try and compensate by going to bed early the next evening, but only go to bed only when you feel sleepy.
If you’re very sleepy during the day, a short 20 minute nap in the early afternoon can help improve your mood and energy without disturbing night time sleep.
Try to keep your wake up time and bedtime as regular as possible - this helps to improve sleep quality, as well as your overall health.
Use your bed for sleeping, and intimacy, and nothing else. Avoid staying awake for long periods in your bed. Instead, if you can’t sleep, go and find a cosy place to read or relax somewhere else in your home, and only get back into bed when you feel sleepy again.
What else can you do to combat menopausal night sweats?
● Regular physical activity is good for sleep quality, and can also help to reduce hot flushes and night sweats. Choose a form of exercise you enjoy, so that you want to do it regularly. Low intensity exercise such as yoga and tai chi, strength based training and moderate intensity cardio have all been found to improve sleep quality.
● Even if you can’t reduce the frequency of night sweats, you may be able to improve your ability to cool down and fall back to sleep. Remind yourself that night sweats are natural - it’s not your fault. Practice taking slow calming breaths to bring your heart rate down and help you to relax.
● Keep a diary of your night sweats and hot flushes to see if you can spot any patterns. Were they related to feeling more stressed, smoking, or having caffeine, for example? Once you recognise your triggers it may be easier to manage them. A diary will also make it easier to check how effective interventions are.
● Your doctor may prescribe hormone therapy or other medications which can help to reduce the intensity or frequency of night sweats.
For most women, night sweats will gradually ease over time, but they can be very persistent. If you’re struggling to cope with night sweats, or if you’ve noticed other symptoms such as a cough, weight loss or fatigue, speak to a medical professional for advice.
This article was written for Bensons for Beds Sleep Hub, and appears here.