Consistent, optimal sleep duration supercharges brain

How well people function has a lot to do with the amount of sleep they get, and a recent study reports that clocking 7 hours of sleep regularly is most favourable for brain performance.

“Substantial changes in sleep duration over time [a]re associated with higher risks of cognitive impairment,” according to a team of researchers from Asia.

Already, a glut of evidence points to long and short sleep hours as bad for cognitive health. The current study adds to this and suggests that cognitive function flags when people make extreme changes in their sleep duration, either by shortening (≤5 hours per day) or prolonging it (≥9 hours per day), the researchers said.

The analysis included 16,948 men and women aged 45–74 years at baseline (1993–1998) from the Singapore Chinese Health Study cohort. All participants underwent assessments at baseline, second follow-up (2006–2010; 12.4 years after baseline), and third follow-up (2014–2016; 19.7 years after baseline).

At baseline, 8.2 percent of the participants were sleeping ≤5 h/day, while 24.1 percent reported 6 h/day, 34.8 percent had 7 h/day, 27.4 percent had 8 h/day, and 5.5 percent had ≥9 h/day. Over an average of 12.4 years, there was a slight increase in the number of those who reported short sleep (≤5 h/day) and a surge in the proportion of those who had long sleep (≥9 h/day).

Participants who were sleeping ≤5 or ≥9 hours per day were more likely to be women, older, less educated, physically inactive, and have worse general health status than those who had 7 hours daily (recommended duration). [J Affect Disord 2020;281:125-130]

When cognitive function was examined at the third follow-up using the Singapore-Modified Mini-Mental State Examination (SM-MMSE), 14.4 percent of the population had cognitive impairment. There was a trend for a U-shaped relationship between baseline sleep duration and risk of cognitive impairment.

With respect to changes in sleep duration, participants with persistently long sleep hours had 50-percent greater odds of having cognitive impairment at the third follow up compared with those who reported having 7 hours per day of sleep at both baseline and the second follow-up (odds ratio [OR], 1.50, 95 percent confidence interval [CI], 1.04–2.16). This risk increase was not seen among participants who maintained short sleep duration.

Meanwhile, participants who increased sleep hours from short or recommended duration at baseline to long duration at the second follow up were at higher risk of cognitive impairment (OR, 2.18, 95 percent CI, 1.37–3.45 and OR, 1.55, 95 percent CI, 1.20–2.02, respectively). The highest risk was observed among those who shortened sleep from long to short duration (OR, 2.93, 95 percent CI, 1.35–6.34).

“Our results suggest that long sleep duration had a bigger impact on cognitive function if the time interval of measurement between sleep duration and cognitive function became shorter, which is consistent with a previous meta-analysis,” the researchers said. [Sleep Med 2016;17:87-98]

“[Furthermore], short sleep may be a marker of cognitive impairment rather than a risk factor because only short sleep at the third follow-up was significantly associated with increased odds of cognitive impairment,” they pointed out.

There are several explanations why sleeping long or short hours hurt cognitive performance, according to the researchers. Mostly, sleep factors in the clearance of metabolic waste from the brain, and so short sleep hours may exact negative effects. [Science 2013;342:373-377; JAMA Neurol 2014;71:971-977]  

Extreme sleep duration might also be a marker of poor sleep quality and, in turn, contribute to brain ageing. It can promote chronic inflammation, up the risk of cardiometabolic disease, and lead to circadian dysfunction, which is associated with degenerative disease. [J Am Geriatr Soc 2014;62:1073-1081; Sleep 2013;36;1027-1032; Lancet Neurol 2019;18:307-318; Lancet Neurol 2014;13:1017-1028]

Despite the presence of limitations, the present data highlight the importance of maintaining optimal sleep duration in the prevention of cognitive impairment, the researchers said.