For decades we’ve been told eight hours of sleep is the sweet spot for brain health. It’s true that snoozing too little, or too much, is linked to increased risk of developing neurological disorders such as Alzheimer’s. But a comprehensive new study from the University of Oslo suggests that for many people, eight hours is overdoing it.
The research team examined brain scans of more than 47,000 people. They looked for a possible link between sleep time and brain size—a widely used proxy for brain health—but found none. Shorter sleep times were not associated with smaller brain volume. In fact, brain size peaked at 6.5 hours of sleep—a bit less than the seven-hour minimum recommended for adults by the New Zealand Ministry of Health.
The researchers came at the puzzle from different angles—tracking 4,000 people over time, and analysing a further 30,000 to focus on those genetically predisposed to sleeping less. They kept getting the same result: shorter sleep time did not shrink brains.
In a growing body of evidence, seven hours has emerged as both the average snooze-time and the ideal for overall wellbeing. But ultimately, “we don’t have a magic number”, says Dr Karyn O’Keeffe, a sleep physiologist from Massey University. “Sleep needs vary from person to person, life stage to life stage, even night to night.”
And length of slumber is only one element, says O’Keeffe. Quality, regularity, and the time of day we choose to hit the hay all contribute to healthy sleep habits. She recommends experimenting to figure out what makes you feel best.