Catnap, kip, snooze, siesta; whatever you call naps, there is no doubt these once frowned-upon short sleeps are gaining acceptance. The increase in popularity is not surprising, with the Centers for Disease Control and Prevention in the US finding around a third of American adults do not get the recommended seven hours sleep each night.
Insufficient sleep not only affects our overall performance, but can affect some physiological functions such as changes to hormones, metabolic factors and immunity. From a business perspective, insufficient sleep can translate into lost profits due to decreased worker productivity. This has led companies such as Google, Nike and Ben & Jerry’s to encourage or allow napping at work, providing employees with napping facilities such as napping pods and quiet rooms in which they can nap if desired.
The pros and cons
Naps have been shown to be effective in reducing and minimising some of the negative effects of insufficient sleep. For example, compared to when no nap is taken, naps have been shown to effectively reduce feelings of sleepiness and improve cognitive performance on tasks such as reaction time and vigilance. Naps may also help to improve short-term memory and overall mood.
Moreover, these improvements can last for a few hours after the nap has ended. Naps may also offer longer lasting improvements in cognitive performance and reduced sleepiness than other commonly used countermeasures of sleepiness such as caffeine.
But as with everything, there are downsides too. Although naps are associated with performance improvements and reduced sleepiness, these benefits may not be immediate. Naps can be associated with a period of sleep inertia, which is the feeling of grogginess most people experience immediately after waking.
Sleep inertia is also characterised by a decrease in performance ranging from slowed reaction time to decreased coordination.
While the effects of sleep inertia generally subside within 15-60 minutes after waking from a nap, this period of delayed responsiveness and grogginess may pose serious risks for individuals who are required to function at optimal levels shortly after waking, such as those in transportation, aviation and medicine.
Following a nap, a period of sleep inertia may occur, before sleepiness is reduced and performance improved.
There is some research showing naps may affect your ability to get to sleep at night. Following an afternoon or evening nap, night time sleep duration may be shortened and more disrupted according to some studies. But there is some debate about this. A majority of the research suggests naps have minimal impact on night time sleep.
It’s all about timing
The degree to which naps help, or hinder, largely depends on the timing and duration of the nap. Longer naps (two hours or longer) are associated with longer lasting performance improvements and reduced sleepiness than short (30 minutes or less) or brief naps (ten minutes or less). Longer naps, however, are also more susceptible to sleep inertia, with a worsening in performance immediately following the nap. Alternatively, the benefits of brief naps occur almost immediately and are without the negative side-effect of sleep inertia.
Longer naps may also have a greater impact on subsequent sleep periods than shorter naps, as they may decrease “sleep pressure”, which can make falling and staying asleep more difficult.
The time of day naps occur can also affect the benefits of napping. Naps taken in the early morning hours, when there is a high circadian drive for sleep, may worsen the effects of sleep inertia and may not offer as much recuperation compared to naps taken in the afternoon.
One sleep or two?
More recently it has been suggested that perhaps humans were not meant to have one sleep, but were meant to sleep bi-modally – two shorter sleeps instead of one long one a day. While there is still some debate about whether this is true or not, it seems the number of sleep episodes may not make much difference to waking performance.
Rather, the overall amount of sleep per day, seven to nine hours, is what is likely to have the biggest impact on performance. It’s possible splitting the sleep in this manner may affect different sleep stages such as non-rapid eye movement and rapid eye movement sleep, which may have long-term implications on general health and well-being, however these effects need to be investigated further.
While there are some disadvantages to napping, such as sleep inertia, for the most part, the benefits of improved performance and reduced sleepiness outweigh the negatives. Short naps, less than 30 minutes, may offer the most “bang for your buck” as they can improve performance quickly with minimal side-effects.
Around a third of the population have trouble sleeping, including difficulties maintaining sleep throughout the night. While night time awakenings are distressing for most sufferers, there is some evidence from our recent past that suggests this period of wakefulness occurring between two separate sleep periods was the norm.
Throughout history there have been numerous accounts of segmented sleep, from medical texts, to court records and diaries, and even in African and South American tribes, with a common reference to “first” and “second” sleep. In Charles Dickens’ Barnaby Rudge (1840), he writes
He knew this, even in the horror with which he started from his first sleep, and threw up the window to dispel it by the presence of some object, beyond the room, which had not been, as it were, the witness of his dream.
Anthropologists have found evidence that during preindustrial Europe, bi-modal sleeping was considered the norm. Sleep onset was determined not by a set bedtime, but by whether there were things to do. Historian A. Roger Ekirch’s book At day’s close: night in times past describes how households at this time retired a couple of hours after dusk, woke a few hours later for one to two hours, and then had a second sleep until dawn.
During this waking period, people would relax, ponder their dreams or have sex. Some would engage in activities like sewing, chopping wood or reading, relying on the light of the moon or oil lamps.
Ekirch found references to the first and second sleep started to disappear during the late 17th century. This is thought to have started in the upper classes in Northern Europe and filtered down to the rest of Western society over the next 200 years.
Interestingly, the appearance of sleep maintenance insomnia in the literature in the late 19th century coincides with the period where accounts of split sleep start to disappear. Thus, modern society may place unnecessary pressure on individuals that they must obtain a night of continuous consolidated sleep every night, adding to the anxiety about sleep and perpetuating the problem.
Less dramatic forms of bi-phasic sleep are evident in today’s society, for example in cultures that take an afternoon siesta. Our body clock lends itself to such a schedule, having a reduction in alertness in the early afternoon (the so-called “post-lunch dip”).
In the early 1990s, psychiatrist Thomas Wehr conducted a laboratory experiment in which he exposed a group of people to a short photoperiod – that is, they were left in darkness for 14 hours every day instead of the typical eight hours – for a month.
It took some time for their sleep to regulate but by the fourth week a distinct two-phase sleep pattern emerged. They slept first for four hours, then woke for one to three hours before falling into a second four-hour sleep. This finding suggests bi-phasic sleep is a natural process with a biological basis.
Pros and cons
Today’s society often doesn’t allow for this type of flexibility, thus we have to conform to today’s sleep/wake schedules. It is generally thought a continuous seven to nine-hour unbroken sleep is probably best for feeling refreshed. Such a schedule may not suit our circadian rhythms however, as we desynchronise with the external 24-hour light/dark cycle.
To successfully maintain a split sleep schedule, you have to get the timing right – that is commencing sleep when there is a strong drive for sleep and during a low circadian point in order to fall asleep quickly and maintain sleep.
Some of the key advantages of a split sleep schedule include the flexibility it allows with work and family time (where this flexibility is afforded). Some individuals in modern society have adopted this type of schedule as it provides two periods of increased activity, creativity and alertness across the day, rather than having a long wake period where sleepiness builds up across the day and productivity wanes.
In support of this, there is growing evidence suggesting naps can have important benefits for memory and learning, increasing our alertness and improving mood states. Some believe sleep disorders, like sleep maintenance insomnia, are rooted in the body’s natural preference for split sleep. Therefore, split sleep schedules may be a more natural rhythm for some people.
Implications for shift work
Split sleep schedules have recently begun to emerge as a potential alternative to continuous night shift work. Working at night has the combined problems of prolonged wakefulness (often working eight to 12 hour shifts) and circadian misalignment (working at a time of night when you would normally be asleep). Shift workers frequently complain of fatigue and reduced productivity at work and they are at increased risk for chronic disease such as obesity, type 2 diabetes and heart disease.
Some industries have employed schedules with shorter, but more frequent sleep opportunities on the premise that the drive for sleep will be less with reduced time. For example, six hours on/six hours off, four hours on/eight hours off, and eight hours on/eight hours off, limit time on shift and reduce extended periods of wakefulness. Split sleep/work schedules divide the day into multiple work/rest cycles so employees work multiple short shifts, broken up with short off-duty periods every 24 hours.
Split-shift schedules that maintain adequate sleep time per 24 hours may be beneficial for sleep, performance and safety. A number of recent studies have found split sleep provides comparable benefits for performance to one big sleep, if the total sleep time per 24 hours was maintained (at around seven to eight hours total sleep time per 24 hours).
However, as might be expected, performance and safety can still be impaired if wake up and start work times are in the early hours of the morning. And we don’t know if these schedules afford any benefits for health and reduce the risk for chronic disease.
While the challenges of night shift work cannot be eliminated, the advantage of some split shift schedules is that all workers get at least some opportunity to sleep at night and do not have to sustain alertness for longer than six to eight hours.
Although we aspire to have consolidated sleep, this may not suit everyone’s body clock or work schedule. It might in fact be a throwback to a bi-model sleep pattern from our pre-industrial ancestors and perhaps work well in a modern industrial setting.
Melinda Jackson, Senior Research Fellow in the School of Health and Biomedical Sciences, RMIT University and Siobhan Banks, Senior Research Fellow, Centre for Sleep Research, University of South Australia