First the easiest part of the question. Do we sleep less in the spring? A lot of research appears to have been done on this, but it does not paint a clear picture. Some studies showed an effect, others did not. “That is partly because those studies often work with questionnaires on which people have to fill in their sleeping times themselves,” says somnologist Laurien Teunissen of the St. Antonius Hospital in Nieuwegein.
But a study which came out last year, did things differently. The American researchers followed 216 Americans on more than thirty nights spread over the year and put them to sleep with a portable device (wearable) Which the sleep duration. What seems? The participants slept 25 minutes less in the spring than in the winter and 12 minutes less in the summer. This difference cannot be explained by later bedtimes. In fact, the participants went to bed on average two minutes earlier in the spring than in the winter, and there was no difference in the summer. “People apparently tend to wake up earlier in the spring and summer,” says somnologist Sebastiaan Overeem of the Center for Sleep Medicine Kempenhaeghe.
wake-up
What causes the slightly shorter sleep time? Days during the American summer have 3.5 hours more daylight than during the winter. In the spring that is 3.6 hours. “So while the impact on sleep duration is limited by comparison, daylight probably plays a role. Daylight has an important influence on the biological clock,” says Overeem. “It promotes wakefulness.”
The seasonal sleep pattern is even stronger in peoples living without artificial light. In 2015, American researchers investigated using wearables the sleeping habits of the San from Namibia, the Hadza from Tanzania and the Tsimane from Bolivia. The Tsimane and San, both living far enough from the equator to have seasonal effects in the amount of daylight, slept nearly an hour longer in winter than in summer.
It was striking that in the summer the San woke up on average one hour after sunrise, while the Tsimane woke up 1.4 hours before sunrise. There is hardly any difference in day length in the habitats. The summer in the San residential area has 11-hour nights and 13-hour days. At the Tsimane, the nights last 11.1 hours and the days 12.9 hours. The researchers therefore attribute the difference to temperature, and not to the amount of light. The habitat of the San is on average 6 degrees colder in the morning.
Then the need for sleep. Do people sleep less in the spring and summer because they need less sleep? “This is a logical, but difficult question,” says Overeem. “Because how do you define ‘need’?” He points out that you then have to look at daily functioning and study whether this remains unchanged in the spring or summer, despite shorter nights. “That is a difficult experiment to set up because of a lot of individual variations. What makes it even more difficult is that people probably already sleep less than is good for them anyway thanks to artificial light and the 24-hour society. Today, your employer probably has a greater influence on your sleep time than the season.”
Still, Overeem dares to speculate. “Indirectly, the data from the American study points to a somewhat lesser need for sleep: going to bed late can be controlled purely by someone’s behavior, but here people woke up earlier, perhaps because the brain has already had enough sleep.”