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The amount of sleep you need to prevent cognitive decline may depend on whether you’re an early or late riser, according to new research. Fiordaliso/Getty Images
  • A lack of sufficient sleep is associated with cognitive decline in a new study.
  • The study reports that people who wake early in the morning require five to six hours of sleep to remain cognitively healthy.
  • The findings also show that people who wake up later require seven to eight hours of sleep.
  • Some data suggests that people who thrive on less sleep may not experience adverse cognitive effects.
  • Short naps may be helpful and may provide short-term cognitive benefits. Longer naps may cause a short-term reduction in cognitive performance.

A new study reports an association between the amount of sleep and eventual cognitive decline.

The findings suggest the influence of sleep duration on a person’s risk of cognitive decline may depend on whether they are an early or late riser.

For the purposes of the research, people were categorized according to the time of day based on their preferred wake-up time. They were classified as “morningness” or “eveningness” types, with an additional subgroup of people expressing no preference, referred to as “intermediate” sleepers.

The study is based on Korean Community Health Survey (CHS) data. It encompassed the self-reported sleeping habits of 224,714 participants. Of these people, 124,990, or 55.6%, fit the study’s morningness type. Just 11,297, or 5.0%, were eveningness sleepers. The remaining 79,409 participants were considered to be in the intermediate group.

When sleep quality was not considered, the researchers found that sleeping seven to eight hours is associated with the lowest risk of cognitive decline.

When the study factored in sleep quality, the lowest risk of cognitive decline was associated with five to six hours of sleep for the morningness group, six to seven hours for the intermediate group. For the eveningness group, it was seven to eight hours.

The study is published in the Journal of Affective Disorders.

Chronotype is typically assessed according to a person’s response in the Munich Chronotype Questionnaire.

Sleep expert Dr. Jonathan Cedernaes at Uppsala University in Sweden, not involved in the study, explained the concept of sleep chronotypes to Medical News Today:

“Typically, our morningness-eveningness type refers to our diurnal preference, a way of assessing our so-called chronotype. Due to natural genetic variation in the population and how we respond to environmental factors such as natural and artificial light, we have differences in when we prefer to sleep versus be active.”

Dr. Ji-Eun Park, the study’s senior investigator, noted: “My study used people’s habitual sleep onset/wake-up time instead of their morningness/eveningness preference since the dataset I used did not include the [questionnaire] variable. So, in my study, morningness/eveningness could be interpreted as their habitual sleep onset/wake-up time schedule.”

Dr. Cedernaes noted a person’s chronotype often changes with age, as older people tend to gravitate more toward the morning hours.

“Many studies have noted differences in risk for various health conditions based on our chronotype or preferred sleep-wake times,” he said. “Part of that is likely a result of differences in exposure to social jet lag, i.e., in part due to how, for example, our work schedules can align with our preferred sleep-wake times.”

Still, Dr. Cedernaes pointed out that measuring sleep duration is somewhat limited, considering everyone has different needs for getting quality sleep.

Sleep expert Jeff Kahn noted another limitation: the study is based on self-reporting. Kahn was not involved in the research.

“Without more objective measures like polysomnography or actigraphy, self-reporting cannot accurately capture detailed sleep patterns, such as the amount of sleep latency an individual has or the number and duration of awakenings,” Kahn said.

Some people appear to do well with less sleep, although its long-term effects are not well-studied.

“I would say that on the one hand, we do not know enough about those who are so-called natural short sleepers,” said Dr. Cedernaes.

“On the other hand, there is encouraging data to suggest that those who get along well on five or so hours of sleep, do not, in general, show any cognitive deficits,” he added.

“Rather, they are often very well-functioning individuals. There is, nonetheless, some contrary evidence,” Dr. Cedernaes noted.

Dr. Cedernaes also said that such mutations in sleep needs are uncommon and often run in families.

Nonetheless, Dr. Park felt, “Even after considering those factors, sleeping less than five hours could be harmful because the result of my study showed a higher risk of five or less sleep hours compared to longer sleeping hours.”

Regular naps, particularly if they are a response to insufficient nocturnal sleep, may indicate poor sleep quality or an inadequate sleep schedule, potentially affecting cognitive performance,” pointed out Kahn.

At the same time, Dr. Park cited one meta-analysis that reported afternoon naps benefit cognitive performance.

Dr. Cedernaes noted, “Short naps can aid learning or memorization.”

He said that longer naps or those taken late in the day may reduce short-term cognitive performance. Dr. Cedernaes described this as “sleep inertia.” Naps may also make it harder to sleep at night.

One night’s sleep typically includes four or five sleep cycles, with each sleep cycle lasting between 90 and 110 minutes.

“Sleep serves a lot of functions, and it occurs in cycles,” said Dr. Cedernaes.

The health benefits afforded by good sleep may be less than optimal when these cycles are interrupted. This may occur more often during daytime sleep and for people who work late shifts, which often “impairs or mistimes sleep,” said Dr. Cedernaes.

Interrupted sleep has been linked to an increased risk of Alzheimer’s disease (AD) and cardiovascular diseases.

High-quality sleep “facilitates critical brain processes such as waste clearance, memory consolidation, and cognitive function… Sleep serves restorative functions for the brain,” Kahn said.