Once you have learned how a night is structured, you quickly understand that the word “sleeping through the night” should not really exist.
Although we have the impression that we sleep through the night, precise measurements show us that we are awake for seconds or even minutes at a time, even during a supposedly “well-rested” night. The decisive factor is how intensively we perceive and interpret these waking phases. There is a great deal of leeway: sometimes a measured minute awake feels like a whole hour – especially in the case of sleep disorders such as insomnia. However, we usually don’t even notice short periods of wakefulness, especially in the first part of the night when sleep pressure is still high and we experience a lot of deep sleep.
The “wolf hour”
Are you familiar with the phenomenon of regularly waking up around 2-3 o’clock in the morning? This phase is often referred to as “wolf hour” – although of course not only wolves are awake at this time (so are bats, hedgehogs, owls and a large part of the population). The name is intended to emphasize how lonely and alert this time feels.
However, the time has less to do with the time of day than with our sleep cycles: if we go to bed at 11 p.m., by 2 a.m. we have already completed two cycles and most of our deep sleep. From now on, we sleep more easily, wake up more quickly and may start to ruminate. At such times, problems seem bigger, the worry about getting enough sleep and performance the next day increases – all of which makes it more difficult to fall asleep again.
In addition to these thoughts, our “internal clock” comes into play – a complex system of hormones that work in a 24-hour rhythm. These hormones, particularly melatonin, cortisol and serotonin, influence us at night. In the wolf’s hour, melatonin is still falling, cortisol and serotonin are at a low point and only gradually rise again, which can affect our mood during this hour of the night.
A look outside the box
At the Arctic Circle, at around 66 degrees north latitude, the conditions go far beyond a normal night: there are periods here when the sun does not set for days (midnight sun) or does not rise (polar night). But how does this affect our internal clock?
Animals such as reindeer have been shown to lack a circadian rhythm in constant daylight. This rhythm is also weakened in other Arctic animals such as ptarmigans, as shown by the measurement of melatonin. Interestingly, the Arctic ground squirrel maintains its circadian rhythm despite the extreme conditions. People in subarctic regions often experience later bedtimes and increased fatigue during the dark season, which can affect their mood, health and stress levels. These seasonal changes can therefore also affect our perceived quality of sleep.
Back to “sleeping through the night”
The concept of “sleeping through the night” should definitely be reconsidered. It can create an unrealistic expectation that puts us under pressure. Moreover, sleep biology is more exciting and varied than just being “awake” or “asleep”. There are exciting intermediate states in which we fluctuate between sleep and wakefulness, as well as phases of “localized sleep” in which parts of the brain are asleep while others are more awake.
Caroline Lustenberger and I have compiled many more secrets of sleep for you in our book.
References
Van Oort, B. E. H. et al. Circadian organization in reindeer. Nature, 2005
Reierth, E., Van’t Hof, T. J. & Stokkan, K.-A. Seasonal and Daily Variations in Plasma Melatonin in the High-Arctic Svalbard Ptarmigan (Lagopus mutus hyperboreus ). J Biol Rhythms, 1999
Stokkan, K. ‐A. & Reiter, R. J. Melatonin rhythms in Arctic urban residents. Journal of Pineal Research, 1994
Williams, C. T., Barnes, B. M., Yan, L. & Buck, C. L. Entraining to the polar day: circadian rhythms in arctic ground squirrels. J Exp Biol, 2017
Friborg, O., Rosenvinge, J. H., Wynn, R. & Gradisar, M. Sleep timing, chronotype, mood, and behavior at an Arctic latitude (69°N). Sleep Medicine, 2014