How harmful is irregular sleep or shift work, and what can you do about it?

Shift work and night work carry multiple health risks. Meta-analyses show that shift workers have a 23% higher risk of coronary heart disease and an elevated risk of type 2 diabetes (relative risk 1.09 to 1.40), although individual studies are not always consistent on this point. For stroke there is a slightly elevated risk (relative risk 1.05), and there is also an association with cancer, but that evidence is mixed and uncertain. Weight gain and a higher likelihood of traffic and workplace accidents have also been reported. All these associations have been established in observational research: cause and effect are therefore not firmly proven, but the patterns are consistent enough to take seriously.

Regarding sleep itself, the effect is clearer. Night shifts and early morning shifts demonstrably lead to substantial sleep deprivation: on average only five to six hours of sleep, with the transition from an evening shift to an early day shift producing the shortest sleep time (an average of 5.20 hours). Both subjective sleepiness and objectively measured reaction times are worst at the end of a night shift. Notably, shift workers after their first night shift believe they are more alert than on later night shifts, yet objectively they perform equally poorly across all night shifts. Working precisely at the moment when the body is biologically in its deepest nocturnal mode -- the melatonin peak -- amplifies this effect further.

Over the longer term there are also risks for mental health and biological ageing. In a large Chinese study (more than 9,000 nurses), more than half of the shift workers reported symptoms of depression or anxiety. Factors such as fatigue, stress around night shifts and limited opportunities for breaks were strongly associated with this. Those who had little influence over their own shift scheduling had a significantly higher likelihood of burnout. Nurses who worked night shifts very frequently (more than 66 times in six months) or who consistently had fewer than 28 hours off between shifts had a 70 to 89% higher chance of a physician-diagnosed sleep disorder, with a clear dose-response relationship. In a large UK Biobank study (195,000 participants), shift work was moreover associated with accelerated biological ageing, with the highest risk seen in irregular and permanent night shift workers.

What can you do about it? Unfortunately, very few interventions have solid evidence behind them. The most concrete measure with moderate quality of evidence is limiting the maximum shift length to 16 hours: this yielded on average nearly three quarters of an hour of extra sleep per day and measurably less on-shift sleepiness. Forward shift rotation -- rotating from day shift to evening shift to night shift rather than the reverse -- appears to reduce on-shift sleepiness, but the evidence is very uncertain. Faster shift rotation (switching shift type more frequently) appears to slightly shorten sleep duration, though here too the quality of evidence is low. A Cochrane review concludes outright that for most scheduling adjustments, including shorter shifts, compressed working weeks and greater employee input, insufficient high-quality evidence exists. There is an urgent need for better studies before strong recommendations can be made.