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Hello, and welcome back to Everyday Science.
Probably like many, my sleep has been suffering in this week’s heatwave. To my embarrassment, I was struggling to keep my eyes open in a post-lunch meeting yesterday.
The intense weather is set to last for the next few days, and even next week temperatures will still be in the mid-twenties in the south of England. So, I’ve been speaking to some experts about exactly how higher temperatures affect sleep, and how the body responds to a longer period of going without enough shuteye.
There are several reasons it is harder to sleep in a heatwave. We may feel sticky and uncomfortable. Open windows may mean more noise from traffic or the dawn chorus. They may also mean a lighter bedroom, especially with the early sunrises of summer.
But higher temperatures also directly interfere with sleep. That’s because core body temperature – in the day, usually tightly held between 36.5°C and 37.5°C regardless of weather – falls by as much as 1.5°C when we sleep.
We don’t know exactly why that happens, says Dr Alanna Hare, a sleep specialist at the Royal Brompton Hospital in London and co-president of the British Sleep Society. But some other bodily processes are turned down during sleep – for instance, heart rate slows and blood pressure falls. “It’s probably a sort of powering down; there are other metabolic processes that pause at nighttime.”
While an overnight dip in body temperature may not seem that surprising, the fall begins two to three hours before our usual bedtime, often in advance of feeling tired – as if it helps to prepare the body for what is about to happen.
“Before we fall asleep, blood flow increases to our hands and feet to help us lose heat,” says Dr Arash Beizaee, a building energy scientist at Loughborough University. “If the bedroom remains too warm, then the body really struggles to cool itself. “That makes it harder to fall asleep and makes it easier to wake up during the night.”
After the first night
The tropical nights – when the air temperature stays over 20°C – that most of England and Wales are currently getting, don’t just reduce the total time spent in sleep but also lower its quality.
Under normal conditions, the brain cycles through several different stages from light to deep sleep, interspersed with periods of rapid eye movement (REM), which is when most vivid dreaming occurs. We tend to have several bouts of REM each night, although we may not remember all our dreams.
It is during REM that the brain seems to consolidate memories and strengthen the ones that are most important. “Your brain figures out what’s gone on all day and what things it needs to note down,” says Dr Hare.
But when sleep is interrupted, either because of the heat, or anything else, this stops the brain from cycling through these stages as it should. The upshot is that less time is spent in REM and less time spent in deep sleep. “It is more difficult to achieve that deep restorative sleep,” says Dr Beizaee.
This helps explain why after a bad night we don’t only feel more tired physically, but are also worse mentally. Our memory and clarity of thinking may be affected. We may also be more emotionally unstable, says Dr Hare: “You move quickly between mood states, and there’s less control over that; you might be more grumpy, but you also might be more tearful.
After several nights
In a heatwave that lasts for several days – as with the current one – the effects of these bad nights add up to create a cumulative “sleep debt”.
Opinions are divided on taking daytime naps to cope. For those trying to beat insomnia where there is no clear external cause, it is generally recommended to avoid daytime napping, to help make sure you feel tired enough at night.
But naps can be helpful for staying sharp in the daytime when sleep has been disturbed by the heat, says Dr Victoria Garfield, an researcher into ageing based at University College London. “We know it is good for our thinking skills.” she adds.
Dr Hare is in favour of daytime naps, as long as they are short and don’t go on beyond 4pm. “Keep it to 20 to 30 minutes – there’s really good evidence that that’s all you need to improve your performance.
“But you don’t want to want to steal from the night. You need a bit of ‘sleep pressure’ in order to fall asleep – and you need to be that bit more tired to get that drive high enough that you can sleep [despite] being hot.”
After a week
We should not worry too much about the longer-term effects of the heatwave. That’s because, when we build up too much sleep debt over several days, the brain changes its progression through the stages of sleep to ensure it gets proportionally more REM.
The brain has more REM cycles through the night and they last longer, a phenomenon called REM rebound. People may notice this because it causes more vivid or strange dreams.
“Your brain will catch up preferentially on the type of sleep it needs first for its function, and then it will catch up on the others,” says Dr Hare.
So, despite my fears, it sounds like my brain will eventually take charge and force a good night’s sleep upon me. “When you’ve had a few nights of sleep deprivation, your sleep need is higher, and that helps you fall asleep and sleep through,” says Dr Hare. “Even though it’s just as hot, you’ll sleep better, and that’s simply your natural sleep drive.”
I’ve also written
You may have heard of good cholesterol and bad cholesterol. Less well known is that there is another type of cholesterol that isn’t affected by statins and isn’t measured by the usual blood tests, but causes thousands of heart attacks and strokes a year.
It is perhaps unsurprising we hear so little about this substance, lipoprotein(a) – also called Lp(a) – as until recently there was nothing we could do to change it. But a new class of medicines that can bring it down are round the corner – and cardiologists are calling them a game changer.
I’ve been watching
Disclosure Day, the new sci-fi movie from Steven Spielberg, has been getting mixed reviews – but I loved it. The premise is that all the alien conspiracy theories are true: extraterrestrials have been visiting the Earth since the 1940s, and sinister US agencies have been covering it up, stealing their technology and even carrying out alien autopsies.
There are a few awkward plot holes, but not enough to spoil the story for me. And it is great to have an alien movie where you really get to see the extraterrestrial beings in full, instead of glimpses and shadows. The aliens did look a little clichéd for my liking, but I suppose that was inevitable given the premise. Colin Firth plays a great tight-lipped villain, and in true Hollywood form, keeps his British accent.
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