Sleep and occupational performance are inseparable: Why occupational therapy practice and research should consider sleep and circadian rhythm

Sleep is essential for human health, and recovery from injury and disease. Inadequate sleep is linked to worse physical and mental health, from increased diabetes risk, to worse cognitive decline (Chaput et al., 2020). Sleep is of particular relevance to occupational functioning; it affects cognitive performance, from alertness and attention, to executive functioning and learning (Tkachenko & Dinges, 2018), and affects physical performance from basic cardiorespiratory functioning to sports performance (Charest & Grandner, 2020). Sleep timing and patterns also affect social inclusion and occupational routines; predictably timed nocturnal sleep is usually preferable, whilst erratically timed or reversed sleep can interfere with people’s ability to make social or work plans (Faulkner & Bee, 2017). Normal sleep is characterised by a cessation of activity, dramatic changes in brain activity and loss of conscious awareness of external stimuli (although the right stimuli will still wake the person). Core body temperature and heart rate drop especially during slow wave sleep (deep sleep), and during Rapid Eye Movement (REM) there is muscle atonia, which prevents people from acting out their dreams (Roehrs & Roth, 2019). Healthy sleep-wake cycles follow a circadian rhythm (as do many other biological processes), this rhythm is set by the circadian clock (in mammals, the suprachiasmatic nucleus, SCN), which controls timed changes in production of neurotransmitters, including a rise in melatonin in the evening before sleep. This internal pacemaker produces an approximately 24h rhythm which continues in the absence of any environmental cues, and which synchronises with the external time of day through regular light exposure (Roenneberg & Merrow, 2016). Sleep pressure, or sleep drive, builds up with longer time spent awake (Milinski et al., 2021), but this sleep drive can be resisted for some time in situations of acute stress. Although this was no doubt useful during our evolution, many modern sources of stress are not improved by remaining awake and vigilant. Stress can lead to short term insomnia. Insomnia may then be maintained by changes to behaviour such as extending time in bed, cancelling daytime activities and coping strategies such as caffeine use (Perlis et al., 2010). Sleep can also be disturbed due to circadian dysregulation; regular daylight exposure and darkness at night is needed for good circadian entrainment, and this can be lacking when spending most time indoors in artificial lighting. Circadian dysregulation does not just affect shift-workers, but can affect teenagers (whose natural rhythm is later), older people (whose natural rhythm can become less robust) and people with particular neuropsychiatric diagnoses which appear linked to poorer circadian rhythm (such as schizophrenia) (Jagannath et al., 2013). Poor circadian entrainment can affect people who because of illness or disability find it difficult to get outdoors on a regular basis, and who may have limited ‘social cues’ (such as work or social commitments) to promote a regular rhythm. Sleep is a relevant consideration for occupational therapists on two levels; firstly, sleep knowledge should inform occupational therapy practice and research across specialities, and secondly, sleep treatment should become more established as a specialism for occupational therapists. Sleep is intrinsically linked to many areas which occupational therapists are well equipped to address, such as the physical and social environment, the timing and type of activities undertaken and optimising performance capacity, see Figure 1. These areas all provide potential targets for occupational therapy interventions for sleep improvement. Furthermore, it is relevant for us to understand the interactions of sleep with these areas, in order to avoid unintended consequences. Some examples of how sleep knowledge is relevant for occupational therapists in ‘non-sleep’ specialities are now explored. Firstly, a person’s performance in assessments may be affected by whether they are sleep deprived, and whether they have recently woken up and are experiencing sleep inertia