As the days stretch long and the sun lingers late into the evening, most of us welcome summer with open arms. Yet for a surprising number of people, this season brings an unwelcome guest: insomnia.
Author
- Timothy Hearn
Senior Lecturer in Bioinformatics, Anglia Ruskin University
For these people, summer is a time of tossing and turning, early waking - or simply not feeling sleepy when they should. Far from just being a nuisance, this seasonal insomnia may chip away at mood , concentration and metabolic health .
But why does insomnia spike in summer - and more importantly, what can be done about it? The answer lies in the light.
Every tissue in the body owns a molecular "clock". However, these clocks take their cue from a central timekeeper - the brain's suprachiasmatic nucleus . This cluster of about 20,000 neurons synchronises the myriad cellular clocks to a near 24-hour cycle.
It uses the external light detected by the eyes as a cue, driving the release of two different hormones: melatonin, which makes us sleepy and a pre-dawn surge cortisol to help us wake.
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In winter, this light cue is short and sharp. But in June and July, daylight can stretch on for 16 or 17 hours in the mid‑latitudes. That extra dose matters because evening light is the most potent signal for pushing the central timekeeper later. In summer melatonin shifts by roughly 30 minutes to an hour later, while dawn light floods bedrooms early and kills the hormone off sooner.
This can have a big effect on the amount of sleep we get. One study monitored the sleep of 188 participants in the lab on three nights at different times of the year. The researchers found that total sleep was about an hour shorter in summer than winter.
Rapid eye movement (REM) sleep - the sleep stage most strongly linked to emotional regulation and the consolidation of emotionally charged memories - accounted for roughly half the sleep loss in summer.
The same team later tracked 377 patients over two consecutive years and showed that sleep length and REM sleep began a five‑month decline soon after the last freezing night of spring. Sleep length shrank by an average of 62 minutes, while REM decreased by about 24 minutes. Slow-wave sleep - the phase most critical for tissue repair, immune regulation and the consolidation of factual memories - reached its annual low around the autumn equinox.
Both studies took place in a city bathed in artificial light - suggesting that even in modern environments our sleep remains seasonally affected.
Big population surveys echo these findings. Among more than 30,000 middle‑aged Canadians, volunteers interviewed in midsummer said they slept eight minutes less than those interviewed in midwinter. The summer interviewees also reported greater insomnia symptoms in the fortnight after the autumn clock change - suggesting the abrupt time shift exacerbates underlying seasonal misalignment .
One study also compared the effect of summer sleep in people living at very different latitudes - such as near the equator, where there's little change in day length in the summer, and near the Arctic circle, where the differences are extreme. The study found that for people living in Tromsø, Norway, their self-reported insomnia and daytime fatigue rose markedly in summer. But for people living in Accra, Ghana (near the equator), these measures barely budged.
This show just how strongly daylight - and the amount of daylight hours we experience - can affect our sleep quality. But it isn't the only culprit of poor summertime sleep.
Temperature is another factor that can spoil sleep during the summer months.
Just before we fall asleep, our core body temperature begins a steep descent of roughly 1°C to help us fall asleep. It reaches its lowest point during the first half of the night .
On muggy summer nights this can make falling asleep difficult. Laboratory experiments show that even a rise from 26°C to about 32°C increases wakefulness and reduces both slow-wave and REM sleep .
Different people are also more vulnerable to summer insomnia than others. This has to do with your unique "chronotype" - your natural preference to rise early or sleep late.
Evening chronotypes - "night owls" - already lean towards later bedtimes. They may stay up even later when it stays bright past ten o'clock. Morning chronotypes, on the other hand, may find themselves waking up even earlier than they normally do because of when the sun rises in the summer.
Mood can amplify the effect. Research found people who suffered with mental health issues were more likely to experience difficulty sleeping in summer.
Chronic anxiety , alcohol use and certain prescription drugs - notably beta blockers, which suppress melatonin - can all make sleep more elusive in summer.
Reclaiming summer sleep
Happily, there are many ways of fixing the issue.
Get some morning sunshine. Try to step outside within an hour of waking up - even if it's just for 15 minutes. This tells the clock that the day has begun and nudges it to finish earlier that evening.
Create an artificial dusk. Around two hours before bed, close the curtains, turn off the lights and reduce the intensity of your phone screen's blue light to help your melatonin rise on time .
Don't let the dawn light in. Being exposed to the dawn light too early will wake you up. Blackout curtains or a contoured eye-mask can ensure you don't wake before you're rested.
Keep things cool. Fans, breathable cotton or linen sheets or a lukewarm shower before bed all help the body to achieve that crucial one-degree drop in core temperature needed to get a good night's sleep.
The deeper lesson here from chronobiology is that humans remain, biologically speaking, seasonal animals. While our industrialised lives flatten the calendar, our cells still measure day length and temperature just as plants and migratory birds do.
By adapting and aligning our habits with those light signals, we might just be able to recapture some sleep - even during the warmer months.
Timothy Hearn does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
