This is an excerpt from an article in this week’s New Scientist
Yet we are surprisingly poor judges of how well we have slept. This even applies long term, and researchers have established that some people develop an “insomnia identity”: more than a third of those who consider themselves insomniacs are, in fact, getting decent sleep. This has led several researchers to ask whether our beliefs and expectations about sleep itself can shape how we feel.
Last year, Samir Akre at the University of California, Los Angeles, and his colleagues tracked 249 people with depression for 13 weeks, taking note of both their actual sleep time using smartwatches and their self-reported sleep behaviour. The mismatch between the two sets of data was striking: many people claimed they had slept terribly, saying it had taken them a long time to get to sleep and that they had woken many times in the night. But the objective data showed that simply wasn’t true.
Notably, when participants performed cognitive tests, it was the self-reported sleep quality that predicted how well they did, rather than the objective sleep metrics. In other words, the belief that they had slept badly impaired their thinking the next day more than the sleep itself.
This is one of several new studies hinting that the way we think about sleep has powerful effects on our subsequent emotions and cognition. In an earlier, slightly mischievous experiment, performed in 2021, researchers allowed 16 adults to sleep for 8 hours on one day, followed by 5 hours the next. Upon waking, a secretly manipulated clock informed the participants how much they had slept. They then rated their subjective sleepiness and took part in a vigilance test that recorded their reaction times.
Those who believed they had slept 8 hours – but who had actually slept only 5 – performed significantly better on the test compared with those who had got 8 hours but thought they had been asleep for 5. Making a person believe they had slept more poorly than they did reduced their reaction times by around 20 milliseconds. This, the team says, is equivalent to the kind of deterioration in performance you would see when someone has four nights of 5 hours’ sleep or two nights with only 3 hours of sleep.
The participants’ brain activity told an even more intriguing story. Slow-wave activity in the brain, called delta power, correlates with what we know as “sleep drive” – the urge to sleep. Delta power is slowest upon waking and increases throughout the day, pushing you to get some shuteye at night. As expected, delta power increased more in the daytime following the 5-hour sleep compared with the 8-hour rest. However, when participants thought they had slept longer than they did, there was a relative reduction in delta power during the day. It appears that simply believing you slept well has a powerful neurological effect, shifting your sleep drive and making you feel more alert for longer.
