A new paper from Cell has shown what many of us know intuitively about sleep: it is about quality, not quantity.
One of the important processes for cognitive renewal that occur during sleep is glymphatic removal. During glymphatic removal, cerebrospinal fluid circulation increases in order to remove waste products from day-time brain activity.
In this 2025 paper published in the journal Cell, Copenhagen-based researchers detected subtle pulses in test subjects’ noradrenaline levels during sleep. The degree of noradrenaline activity correlated with the quality of glymphatic removal in test subjects.
This was understood by the research team to be because noradrenaline causes very slight constriction and relaxation of the vessels that make up the circulatory system in the brain. This pulsing is referred to as “vasomotion”.
When it comes to understanding sleep, the finding has two very interesting aspects to it.
Firstly, the colloquial understanding of sleep is that REM sleep – when dreaming takes place – allows the brain to refresh itself, while during non-REM sleep the body is the focus of repair. Yet the Copenhagen researchers found that noradrenaline pulses occurred during non-REM sleep only. This implies that the physical repair of the brain requires deep sleep, even as other brain functions related to processing of memories and emotions are the focus of REM sleep.
Secondly, the Cell research was carried out with some test subjects falling asleep naturally and others taking sedatives. Sleep science commonly uses sedatives on subjects in order to create a baseline for comparability between their results.
Yet in this research, the noradrenaline pulses in the brain were not seen in patients who had taken sedatives. This suggests that although sleeping pills may seem to trigger long, deep sleep, in fact the level of brain recovery (via glymphatic removal) that takes place is inhibited relative to sleeping naturally.
The long-term effects are unknown, but impaired glymphatic removal is understood to be linked to the build up of metabolic waste products and from there to cognitive decline and degenerative conditions such as Alzheimer’s.
Another recent study found that repairing DNA damage in neurons drives the need for sleep even amongst organisms that have no brains.
It’s another reason to prioritise getting the best, natural sleep you can get.
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