The Average Optimal Amount of Sleep for Brain Health Would Be Considerably Less Than 8 Hours

By analyzing data from thousands of individuals, scientists have discovered that the optimal brain volume is associated with 6.5 hours of sleep. These findings could influence recommendations regarding sleep duration and its relationship to brain health.


Sleep has always been considered essential for physical and mental health. Recently, studies have suggested that sleep deprivation could be linked to diseases like Alzheimer’s and even lead to brain atrophy.

In fact, most health organizations recommend that adults sleep between 7 and 9 hours per night. With the advent of sleep tracking devices, it is easier than ever to determine if one is meeting this recommendation. However, is this really the optimal duration for everyone?

This theory and its connection to potential brain atrophy due to lack of sleep are under scrutiny in a new study by Anders Fjell from the University of Oslo. By exploring the relationship between sleep duration and brain health, the study’s authors, published in the journal Nature Human Behaviour, shed new light on our actual sleep needs, which average around 6.5 hours for a healthy brain volume.

A More Complex Relationship Between Sleep and Health Than It Seems

sleep test
(a) Self-declared sleep duration (green) overlaid on recommended sleep intervals (the blue/gray area). (b) Regional groups showing similar relationships between sleep thickness and duration. (c) Subcortical and global volumes as a function of sleep duration. Image: A. M. Fjell.

The duration of sleep has long been a concern in medical and scientific circles. Previous research has shown a U-shaped curve, indicating that both too short and too long sleep durations could have adverse effects on health.

During sleep, the brain’s clearance mechanism becomes more active, removing harmful substances, including those associated with Alzheimer’s, such as beta-amyloid protein. Large-scale studies have shown that people who sleep too little or too much exhibit signs of deteriorated health, such as memory problems and a decrease in brain volume, typical symptoms of Alzheimer’s disease.

However, it is crucial to note that these studies are based on correlations and not direct evidence of causality. Faced with these ambiguities, Anders Fjell and his team from the University of Oslo decided to delve deeper into the analysis.

Discoveries That Challenge Conventional Wisdom

The methodology employed by Fjell’s team was rigorous. Based on a substantial sample of 47,000 individuals, the researchers observed that the maximum brain volume corresponded to a sleep duration of 6.5 hours, an unexpected result compared to the usual recommendations. Instead of the other way around, the observed correlation might be due to brain atrophy causing sleep disturbances.

Moreover, a longitudinal study involving 4,000 participants over 11 years revealed no link between the initial sleep duration of participants and a decrease in brain volume over time. These findings challenge the popular belief that insufficient sleep could lead to a reduction in brain volume.

The Natural Variability of Sleep Needs

According to Fjell, each individual has a unique sleep pattern, resulting from a combination of genetic, environmental, and physiological factors. This natural variability means that the ideal sleep duration can differ from person to person. The brain has self-regulation mechanisms, called homeostatic mechanisms, to ensure that we get the necessary amount of rest to function optimally, unless circumstances prevent it.

The most reliable indicator of our sleep needs may well be our own feelings. If a person constantly feels exhausted, it’s a clear sign that they are not getting enough rest. Conversely, if they feel energetic and alert throughout the day, it probably means they are meeting their sleep needs correctly, whether it’s 5, 7, or 9 hours per night.

Rather than fixating on a specific number of hours, it’s essential to listen to one’s body and its signals. Also, a useful tip: when adjusting the amount of sleep, it should be done in 90- to 110-minute increments (representing the average sleep cycle time) rather than 60-minute increments.

This allows for optimizing waking time. Ideally and as a general rule, a person should sleep for 6 hours, 7.5 hours, or 9 hours to optimize their waking time (according to their needs). If one is fortunate enough to know their approximate “falling asleep time,” it’s possible to set the alarm for that ideal moment, provided they go to bed on time. For example, a person who requires 6 hours of sleep and typically takes 30 minutes to fall asleep can set the alarm for 6:00 AM and go to bed at 11:30 PM.