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Unlocking the mysteries of sleep

Updated: Mar 30, 2023

Using Forced Desynchrony designs to investigate the impact of circadian rhythms and homeostatic sleep pressure on human sleep.

A blog by Renske Lok


Sleep and wake are tightly regulated by an interplay of the circadian system and homeostatic sleep pressure, more info. Many of our bodily processes including alertness, cognitive performance, physical performance, and sleep, are influenced by both systems. To better understand and design targeted interventions, it is crucial to determine the contribution of each of these systems. For example:

  • Suppose sleep was only driven by homeostatic sleep pressure. In that case, one could fall asleep at any time of day as long as one has been awake for long enough.

  • Simultaneously, suppose sleep was only driven by the circadian system. In that case, one could only fall asleep only when the internal clock dictates to do so.

We now know that sleep, in fact, is driven by a combination of circadian clock phase and homeostatic sleep pressure; one can only fall asleep when one has been awake for long enough, and their circadian clock tells them to do so.



To determine the influence of homeostatic sleep pressure and circadian clock phase, a complicated experimental design is necessary in which one can separate these effects. One of the ways to do so is by conducting a Forced Desynchrony (FD) experiment. FD experiments investigate the impact of the internal clock, or circadian rhythm, and the duration of prior wakefulness, homeostatic sleep pressure on a desired output measure. In an FD experiment, participants are subjected to an artificially imposed sleep-wake cycle that is longer or shorter than 24 hours and, in part, is out of sync with their natural circadian rhythm. For example, in a typical FD experiment, participants might be subjected to a 28-hour day, where they are kept awake for 16 hours, followed by 12 hours of sleep. This means that their sleep and wake times will gradually shift later over time, while the circadian clock free runs, unsynchronized with the 24-hour day.


Researchers can use a variety of measures to study the effects of forced desynchrony on the body. For example, they might monitor the participants' sleep, hormone levels, or alertness throughout the experiment. They can then use mathematical calculations to calculate the contribution of the circadian clock and homeostatic sleep pressure, as they know precisely at which circadian clock time a measurement took place, as well as the amount of build-up homeostatic sleep pressure at that time. One of the key findings from forced desynchrony experiments is that the body's natural circadian rhythm is not precisely 24 hours long. In fact, the average length of the circadian rhythm is slightly longer, closer to 24.2 or 24.3 hours. This means that the body needs to constantly adjust to the timing of external cues in order to stay in sync with the 24-hour day.


FD experiments can also shed light on the effects of disrupted sleep-wake patterns on health and well-being. For example, FD experiments have shown that people who work night shifts and thus are awake when the circadian system promotes sleep may be at increased risk of various health problems, including obesity, diabetes, and cardiovascular disease. Understanding how the circadian clock and homeostatic sleep pressure contribute to these disruptions can help researchers develop better strategies for mitigating these risks.

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