To many people, the phenomenon known scientifically as the circadian rhythm is bleeding obvious. We sleep in the night and are awake during the day, long-haul flights like those from the UK to Australia gives you jetlag, and night shifts are a right pain in the bum. Detailed explanations involving transcription-translation feedback loops and phase response curves don’t change those facts, they’re a fact of life when we live on a rotating world. But many scientists, myself included, are fascinated in the details, and some scientists, like Céline Vetter and colleagues at the Institute of Medical Psychology at Ludwig-Maximilian-University in Munich, use this eye for detail to find out how we might best cope with our biological timing in a 24-hour society.
You are very probably aware of whether your friends and colleagues are morning or evening people, when they are most awake during the day, or how dependent they might be on coffee at certain times of day. A significant proportion of this preference is hardwired within them, the outward sign of their internal circadian clock, and is known as their chronotype. We as a species are broadly diurnal, meaning we’re awake during the day, but there are a range of chronotypes that result in people having there peaks at different times of day. For example, some people are naturally inclined to be morning people – the larks or early birds – and some people are inclined to be evening people – the night owls.
In some professions, people have to override their chronotype. When duty calls, they need to be available. Shift work is the most striking example: there’s no point in an A and E doctor being asleep when an emergency comes in at 3am.
So people force it, delay sleep until convenient times and rely on alarm clocks and stimulants to keep them awake when they need to be. This does not come lightly: more and more research is showing that overriding your natural rhythm can result in increased incidences of cancers, heart problems and neurological disorders, as well as impairing short-term cognitive function (Takahashi et al 2008).
Can anything be done about this? Possibly. It is possible to adjust your circadian rhythm to make it easier to adapt through taking drugs and exposing yourself to bright light at key times (Burke et al 2013). In some situations (like international air travel and jet lag), interventions like these are the only way to speed your adaptation to the new time zone. However, in other situations, like shift work, it may prove more beneficial to change the pattern of shift work in accordance with a person’s chronotype rather than force people to work against their internal clock.
Céline Vetter and colleagues tested this by analysing sleep quality, sleep duration, social jetlag (the time difference between the mid-point of sleep on work days and the mid-point of sleep on free days), and ratings of wellbeing and stress, in people working on three different shift schedules: morning (06:00 – 14:00), evening (14:00- 22:00) and night (22:00 – 06:00). Their first measurements were when people worked an even distribution of all shifts – it didn’t matter if you were a morning person or an evening person, you worked two morning, two evening and two night shifts in a nine day period (with three rest days). They then altered each worker’s shift pattern to reduce the number of shifts that they expected to be most strenuous: morning types had more morning shifts and fewer night shifts; evening-types had fewer morning shifts and more night shifts. They called this the chronotype-adjusted schedule (CTA).
As expected (and maybe somewhat obviously), workers with the most extreme chronotypes benefitted from the adjusted schedule. Eliminating night shifts in early birds and eliminating morning shifts in night owls resulted in more and better quality sleep on workdays, less sleep on free days, improved wellbeing ratings and, for early birds at least, reduced social jetlag. Interestingly, social jetlag does not change in night owls between the normal shift pattern and altered shift pattern – it is likely that the night owls better tolerate night shifts than early birds, and therefore don’t shift their sleep pattern as much between work days and free days.
What can we take from this? Well, whilst interventions such as light treatment and drugs are necessary in some situations, we may be able to avoid the damaging effects of shift work by changing the work rather than the self. Though this might be difficult for employers who require 24-hour productivity, an awareness of chronotype is becoming more necessary in work place health and safety considerations.
Also, it is worth bearing in mind the effect of a social jetlag that affects us all: the transition from normal time to summer time and vice versa at Clocks Forward and Clocks Back. This shift of an hour is actually a form of social jetlag: people shift their behaviour by an hour to match their watch time rather than the sun time, meaning your internal clock is one hour out of sync. This one hour jetlag dramatically affects health: a 10% increase in the incidence of heart attacks has been observed at these times of year (Janszky et al 2012) as well as compromising sleep quality (Lahti et al 2008). There are many economic reasons to switch permanently to summer time; is this effect on health an additional reason to switch permanently to summer time?