How many hours do you need to sleep each night to stay healthy?
The National Sleep Foundation's sleep time duration recommendations are pretty clear. The sleep experts suggest the appropriate sleep duration for teenagers is eight to 10 hours per night, while it is seven to nine hours for young adults and adults, and seven to eight hours for older adults (Hirshkowitz et al., 2015). Thus, they consistently recommend getting seven hours or more of sleep per night, no matter which age group you are in.
The results of a new multinational neuroscience study (Zitser et al., 2020) now question this recommendation. In the study, the authors collected the sleep duration data of 613 participants over an impressive period of 28 years. Data collection took place from 1985 to 2013. Based on their sleep duration, participants were categorized into four different groups: “five hours or less,” “six hours,” “seven hours,” or “eight hours.” Each participant had to perform a number of tests to assess their cognitive abilities.
These tests included a wide variety of cognitive abilities like memory, higher cognitive functions and reaction times. In addition, the researcher used a magnetic resonance imaging (MRI) scanner to assess several aspects of brain structure. These included the structure of the grey matter of the brain, e.g. the areas of the brains that mainly contain the cell bodies of neurons. Moreover, the structure of the white matter of the brain, e.g. the neural connections between different grey matter regions, was assessed.
The authors had the hypothesis that people who consistently slept at least seven hours per night would have improved cognition as well as improved grey matter and white matter structure than those people who consistently slept less than the guidelines recommend.
The results, however, were quite surprising.
In general, they found that for most people sleep duration remained relatively stable over 28 years. About 12.7 percent of the participants slept eight hours, about 45.4 percent slept seven hours, 37.2 percent slept six hours and 4.7 percent slept five hours each night.
Remarkably, there were absolutely no differences in cognition or any of the brain parameters between the different sleep duration groups. Thus, sleeping six or even five hours each night was not associated with impaired cognition or any negative effects on the brain. The authors suggested that it might be more meaningful to also analyze sleep quality when assessing the impact of sleep on cognition and brain structure. Moreover, this suggested that current sleep duration guidelines might need to be redesigned if further studies support their results.
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Hirshkowitz M, Whiton K, Albert SM, Alessi C, Bruni O, DonCarlos L, Hazen N, Herman J, Katz ES, Kheirandish-Gozal L, Neubauer DN, O'Donnell AE, Ohayon M, Peever J, Rawding R, Sachdeva RC, Setters B, Vitiello MV, Ware JC, Adams Hillard PJ. (2015). National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health, 1, 40-43.
Zitser J, Anatürk M, Zsoldos E, Mahmood A, Filippini N, Suri S, Leng Y, Yaffe K, Singh-Manoux A, Kivimaki M, Ebmeier K, Sexton C. (2020). Sleep duration over 28 years, cognition, gray matter volume, and white matter microstructure: a prospective cohort study. Sleep, in press.