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Sleep & health: regularity vs. Duration

Sleep disorders are a set of conditions that affect the rhythm, quality and duration of sleep. They can have significant repercussions on physical and mental health. Insomnia, obstructive sleep apnea, hypersomnia, circadian rhythm disorders and restless legs syndrome are all examples of sleep disorders. Lifestyle, level of physical activity, consumption of alcohol, tobacco or stimulants, as well as anxiety and depression, can all lead to sleep disorders. What's more, numerous studies have shown that both subjective (self-reported) and objective (measured) estimates of average sleep duration are associated with a higher risk of mortality. In fact, too little sleep (< 7 hours) or too much sleep (> 9 hours) is associated with a higher risk of all-cause mortality.

Sleep duration is therefore often at the heart of official health recommendations. However, recent data indicate that sleep regularity, defined as a daily regularity of waking and sleeping schedules, may be a better predictor of certain health problems than average sleep duration. Some longitudinal studies have shown associations between irregular sleep and cardiometabolic health, epigenetic aging, mood disorders and lower quality of life. People with irregular sleep patterns are exposed to irregular environmental stimuli, including light, and may therefore adopt irregular behaviors, such as physical activity and eating. This instability of stimuli and behaviors would lead to the disruption of circadian rhythms, with consequent negative repercussions on health. However, the mortality risk associated with sleep regularity has not been prospectively studied in a large cohort with objective sleep data. So, is the sleep regularity really more important to consider than sleep duration?

The Study

To answer this question, an international team of researchers assessed the relationship between objectively measured sleep regularity and the risk of all-cause, cardiometabolic and cancer mortality. To do this, the researchers used data collected via the UK Biobank, a biomedical database containing information on genetics, lifestyle and health from half a million British citizens.

Of the 502 000 people, aged between 40 and 69, recruited for the UK Biobank between 2006 and 2010, 103,669 participants wore a 3D accelerometer (Axivity AX3) on their dominant wrist for 7 days under free-living conditions between 2013 and 2016. The final analysis included 60,997 participants with valid Sleep Regularity Index (SRI) scores. This index assesses the daily regularity of sleep patterns, taking into account irregularity due to sleep fragmentation, naps and variable start and end times. An SRI of 100 represents perfectly regular sleep-wake patterns, and zero represents totally random patterns. The researchers also assessed whether sleep regularity was a better predictor of mortality risk than sleep duration.

In addition to the sleep variables, the researchers collected information on covariates such as average physical activity, self-reported ethnicity, occupational status, rotational or non-rotational work, annual household income, place of residence, weekly social activities, smoking, use of medication for high blood pressure or cholesterol, diagnosis of cancer, diabetes or vascular disease, body mass index (BMI), cholesterol levels, mood disorders and consultation of a GP or psychiatrist for mental health problems.

Between June 2013 and March 2021, the researchers collected information on participants' mortality. SRI and sleep duration were divided into quintiles. Mortality risks were estimated for each of the four upper quintiles of SRI and sleep duration relative to their respective lower quintiles, which were assumed to have the highest mortality risk.

Results & Analyzes

The main results of this study show that over 10 million hours of accelerometric data in 60,977 UK Biobank participants, more regular sleep is a significant predictor of a lower risk of all-cause mortality. Participants in the top four quintiles (SRI = 71.6-98.5) had a 20-48% lower risk of all-cause mortality than those with SRI scores in the bottom quintile (SRI < 71.6). Sleep regularity was a stronger predictor of all-cause mortality than sleep duration, even after adjustment for the various covariates collected.

This may be explained by the fact that sleep regularity is a more direct indicator of circadian rhythm disruption, which experimental studies show to have wide-ranging adverse effects on physiology. In animal studies, circadian disruption induced by light patterns that contribute to irregular sleep-wake behavior is known to cause premature mortality, and deliberate disruption of the circadian clock by light causes cardiovascular disease. This link has been observed in humans for their cardiometabolic health.

It should be borne in mind that while sleep regularity is a more direct measure of circadian rhythm disruption, sleep duration can also, in part, capture some aspects of it. Short or long sleep duration can influence the timing of light exposure, nutritional intake and physical activity, all of which have an impact on central and peripheral circadian rhythms. In this study, a higher risk of mortality from cardiometabolic causes was associated with irregular and short sleep duration.

For cancer, irregular sleep predicted a higher risk of mortality, whereas short sleep duration did not. As with all-cause mortality, this relationship may be due to circadian rhythm disruption. Several research studies have established a link between circadian disruption and cancer: studies implicating gene expression and metabolic rhythms in cancer initiation and progression, animal studies showing that irregular sleep-wake rhythms and circadian rhythm disruption lead to increased tumor progression and metastasis, and cancer-induced inflammation, and epidemiological evidence implicating night-time light exposure in a higher risk of breast, thyroid and pancreatic cancer, and the link between rotating work and breast, prostate, lung and skin cancer. On the other hand, epidemiological data indicate that sleep duration is linked to a higher risk of cancer mortality only in people who sleep a lot (>9 hours), which is consistent with the results of this study.

Practical Applications

Research into sleep and its impact on health has focused on sleep duration as the main predictive factor. The results of this study confirm the important role of sleep duration in predicting mortality, but reveal that sleep regularity is an even stronger predictive factor. For both psychosocial and biological reasons, extending sleep duration can be difficult to implement in practice. Conversely, asking people to maintain more similar sleep hours from one day to the next to improve sleep regularity, rather than devoting more of the day to sleep, might be a simpler strategy to implement.

Note that people with the highest SRI scores fell asleep and woke up at virtually the same times (within a one-hour window) on the majority of the days the accelerometer was worn. On the other hand, people with the lowest SRI scores fell asleep and woke up in approximately 3-hour time slots. Of course, night-time awakenings and fragmented sleep patterns also contribute to low SRI scores and should therefore be taken into consideration.

References

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