Taking a Personalized Approach to Sleep

In April, a study in the journal Nature Aging reporting that seven hours is the optimal amount of sleep in middle age, made headlines around the world. In this study, shorter and longer durations of sleep were found to be associated with poorer physical and psychological outcomes. The study’s findings were compelling given the high number of participants and the length of the project: during a period of ten years over 500,000 individuals self-reported on their sleep and were given cognitive, mental health and wellbeing scores. Furthermore, approximately ten percent of that cohort were also submitted to genetic testing and brain scans. 

The ‘U’ shaped relationship between sleep duration and mortality/morbidity – with both very short and very long sleep durations being associated with poor health – has been known to sleep experts for many years. Of course, this study alone is not sufficient to confirm the causal relationship between health problems and sleep duration, and does not account for the physical or psychological illness that may be present leading to worse health outcomes including sleep disruption.  It’s also important to note that there is little evidence to suggest sleep duration in the range of six to eight hours being associated with significant harm.

It is difficult to extrapolate findings from large research studies to single individuals. For instance, sleep changes significantly in duration, timing and pattern during our lifespan. As we age, we become more likely to experience fragmented sleep and a tendency to wake earlier in the morning. While newborn babies sleep for up to 18 hours per day – with half of this consisting of REM sleep – from their mid-teens to mid-twenties, most people experience a delay in their circadian rhythm, with night-owl tendencies becoming the most common.  There is then a gradual shift of the biological clock to an earlier set point from the age of 30 onwards which causes young adults to both go to sleep and wake earlier. From mid-life onwards, sleep fragmentation becomes more and more prevalent, affecting sleep quality as well as the proportion of time spent in deep sleep and REM phases, which are essential for physiological and psychological well-being. 

The primary causes of morbidity associated with aging -- cardiovascular disease, metabolic disease, cognitive impairment, and cancer – are indeed all associated with a poor quality of sleep and shorter sleep durations. It’s always difficult, however, to determine whether disrupted sleep is pathogenic or the consequence of disease. Invariably, it’s likely to be both. For instance, it’s clear that mood disorders may have a significant impact on sleep duration that, in turn, worsens the mood disorder. Both need to be managed. There is also a clear relationship between circadian disruption, sleep fragmentation and destabilization of bipolar disorder and schizophrenia. Or consider any health condition that causes pain or the need to use the bathroom overnight – that will clearly disrupt sleep as well. Hence hours of sleep can be a marker of other health concerns and not in itself harmful.

Similarly, longer sleep durations are associated with negative outcomes. However, it is very likely in this case that the number of hours in bed acts as a surrogate for a host of other issues, including physical and mental health disorders and the prescription of sedative or narcotic medications. In such cases, long periods of sleep are unlikely to be the cause. Instead, it’s the underlying problems that cause negative health outcomes and also lead to longer periods of recumbency. There’s also the problem that most population cohort studies rely on self-reporting, which is often interpreted as time spent in bed, rather than time asleep. As such, longer sleep durations might be overestimated. On the other hand, it is also well known that insomnia sufferers tend to report a lower number of hours slept than those that actually occur.

Sleep duration is an important aspect to consider in regard to sleep health but following a prescriptive number of hours will not necessarily address potential problems. The approach of optimizing for a 7-hour duration only in midlife, also misses the nuances of considering co-existing health conditions, quality of sleep, circadian rhythm and timing of sleep which are all essential for efficient recovery.

Most people of working age will have a degree of sleep deprivation, although this is unlikely to have pathological long-term consequences for the majority.  The focus, therefore, should be on ensuring that an individual has sufficient sleep for their lifestyle requirements such that they wake up feeling refreshed and restored with sufficient energy to attend to all of their daily activities without difficulty.  Signs that this is not the case can include a heavy reliance on caffeine or other stimulants, tiredness on waking, sleepiness during the mid-afternoon circadian dip, generalized fatigue and difficulties with regulation of mood and appetite. Ideally a sleep duration of seven to nine hours is recommended for most individuals, but for some, sleep durations outside of this range may be entirely normal.  

Timing sleep to match an individual’s circadian rhythm and the regularity of sleep schedule are important factors in restorative sleep.  Disruption of these patterns can lead to chronic health issues independent of duration of sleep. Individuals who sleep for a sufficient duration but continue to be tired and fatigued during the day (assuming exclusion of other health conditions) should undergo a thorough interrogation of their sleep quality.  Disruption of sleep quality leads to insufficient periods of deep restorative sleep and REM periods, common causes include sleep apnea, movement disorders, poor sleep hygiene and partners who snore loudly.

In Summary, interpreting the conclusions of these large studies as showing that a single individual should follow a prescriptive number of hours does not reflect the whole story. However, the study does reinforce an important message: abnormal sleep in middle age should never be ignored. Taking a personalized approach means that the underlying causes of disordered sleep can be found, and managed, normal sleep duration restored, consideration given to quality and timing of sleep and long-term risk mitigated.