This weekend sees the longest day of the year in the Northern Hemisphere. Here in the UK we’ll get over 16.5 hours of daylight (possibly even some sunshine too!) In the Arctic Circle, the sun won’t set at all. It’s all down to the tilt in the Earth’s axis. But will the longest day of the year affect our sleep? Does our sleep really change with the seasons? The answer is yes and it’s because of the link between daylight and our sleep/ wake cycle.
Exposure to daylight early in the morning can impact our internal biological clock, and so shift the timing of our sleep window. This means that everything gets shifted a little earlier. Under controlled experimental conditions, people actually tend to go to bed earlier and wake up earlier in summer. Although this urge to go to bed earlier may not always translate into reality when we’re out of experimental conditions and watching the World Cup!
The length of time we’re exposed to daylight also affects our sleep/wake cycle changes. One study compared sleep in Norway, where there’s a big seasonal variation in daylight hours, to Ghana, which is close to the equator and so has little variation. Sure enough in Norway, both bed and waking times were found to be earlier in summer, while insomnia, fatigue, and low mood were more prevalent in winter. These winter-summer seasonal differences were just not present in Ghana. Studies have shown that during the summer, we tend to reach core body temperature and the time when we produce melatonin (the hormone associated with sleep onset) earlier in the night compared to in the winter.
There are other environmental factors that can affect our sleep during summer, like the temperature and light levels in our bedroom. We still need to make our bedroom as sleep-friendly as possible, keeping the temperature around 18° and our bedroom nice and dark even if it’s warmer and lighter for longer outside!