The Frontal Cortex

The Night-Shift and Naps

Posted by Jonah Lehrer on March 25, 2008
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I had no idea this many Americans were nocturnal:

Twenty percent of American workers are night-shift workers, and the number is growing by about 3% per year, according to the Bureau of Labor Statistics. While the rest of society sleeps, police officers, security guards, truck drivers, office cleaning crews, hotel desk clerks, nurses, pilots and many others keep patients alive, streets safe and packages moving. But at a price.

These workers — and people with more conventionally sleep-deprived lifestyles — are known to be at higher risk for accidents, sleep disorders and psychological stress due to daytime demands, such as family and other obligations, that interfere with sleeping. Now scientific evidence suggests their disrupted circadian rhythms may also cause a kind of biological revolt, raising their likelihood of obesity, cancer, reproductive health problems, mental illness and gastrointestinal disorders.

The evidence for an increased cancer risk is so compelling that, in December, the International Agency for Research on Cancer, a unit of the World Health Organization, declared that shift work is “probably carcinogenic to humans.”

Natural selection has invested a lot of effort in our circadian rhythms, so it’s a little distressing to learn that 20 percent of us are flagrantly violating all those CLOCK genes.

The news isn’t all bad, however. Over at Mind Matters, we just featured some new research which demonstrates the surprising power of power naps. Even if you can’t get eight hours of sleep during the wee hours of the morning – and people on the night-shift sleep significantly less – you can make up for the lack of sleep with short bursts of non-REM sleep:

In this study, 18 college students participated in three different experimental conditions: taking a long nap (35 minutes), a short nap (six minutes) or remaining awake. Prior to napping (or remaining awake), participants learned a list of 30 words and were tested on these words one hour later. Learning always took place at 1 P.M. and testing took place at 2 P.M.; the question was whether napping would produce better memory retention than wakefulness. The short nap consisted of relatively light sleep, mostly stages 1 and 2 non-rapid eye movement (NREM) sleep. The long nap contained even more of this light sleep, along with about 10 minutes of deeper slow-wave sleep (stages 3 and 4 NREM). Similar to a previous study by Tucker et al. (2006), the long nap resulted in better memory for the words than wakefulness. But, remarkably, so did the short nap, albeit at a lower level. Nevertheless, this suggests that just six minutes of sleep can produce significant memory benefits.

What no one can figure out, though, is why even short naps can have cognitive benefits. After all, the neural mechanisms that underlie the consolidation of new memories are believed to take at least sixty minutes – protein synthesis takes time. What, then, is going on? Jessica Payne and Matthew Walker hypothesize:

One possible answer resides in the electrical signals of sleep. For example, short nappers obtained a few minutes of stage 2 sleep, a stage that produces “sleep spindles.” These powerful synchronous bursts of electrical activity have been linked to memory stabilization. Alternatively, such short naps may not necessarily change the strength of the memory, but rather allow easier access to it. That is, sleep, and just a small amount, may “grease” the recall-shoot that memories slide down, allowing easier access to the material. Yet another idea is that ultra-short naps may rapidly refresh regions of the prefrontal lobe (which suffer the greatest burden when sleep deprived), and, as a result, allow the brain to more accurately search its storehouse of memories.

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Comments

  1. #1 Gunnar
    March 25, 2008

    Interesting read. I have discovered a similar tipping point during my own private “siesta research”, which amazingly (at least to my own amazement) corresponds perfectly with the observed data from the study. When napping for 30 minutes I used to sometimes oversleep and wake up dizzy after 1-2 hours, so by shortening my naps to 25 minutes I now wake up feeling energized, with almost 100% certainty. The 5 minutes shortening, avoiding entering stages 3 or 4, seem to help me get up where I sometimes used to oversleep.

  2. #2 chezjake
    March 25, 2008

    An interesting, if disjointed, post. I fail to see any clear connection between the two parts.

    I have a few thoughts and questions, mainly on the shift-work part. The linked article doesn’t specify how “night shift” was defined for any of the research and the two specific examples were of people who were working close to 12 hour shifts. I’d be interested to know what results/correlations there are between day-shift, evening-shift, and night-shift workers, particularly the latter two. Also, is there any data on people who work rotating shifts?

    I’m also wondering if that 20% combines both night and evening shift workers.

    And here’s an interesting possibility. It’s pretty well known that some people are naturally “not morning people” and some are naturally “night people.” (I’ve never been worth a hoot in the morning, since I was a kid.) I’m wondering if it might help to have people tested early on in life, and then have those who seem to be late-day and night people counseled that they might prefer and do better at occupations where they can work compatible shifts.

  3. #3 Ian
    March 25, 2008

    That’s the problem with media reports – no definitions! I don’t see how they’re defining night shift. Is it literally overnight, or anything starting after 3pm, 5pm, 6pm?

    It would be interesting to compare the proportion of “off-shift” workers in various nations and the levels of social problems those nations have.

  4. #4 Pierce R. Butler
    March 25, 2008

    “shoot” /= “chute”.

  5. #5 Luna_the_cat
    March 29, 2008

    For my last few years at BP, the bastards had me (and several other unfortunates) on the worst possible shift pattern that the sick minds of accountants could devise. Basically, they wanted to cover some equipment 24/7 with as few people as they could manage, without violating the EU directives for how many hours in a row or days in a week a person could be made to work. We had 12-hour shifts, either 8am->8pm or 8pm->8am, and the shift pattern was:

    1 day on
    1 day off
    3 nights on
    3 days off
    2 days on
    1 day off
    4 nights on
    4 days off
    2 days on

    …so as you can see, we rotated through weeks in sync with absolutely nothing, no regular pattern, and without any time to even try to adjust to sleeping day or night. I got all too used to having to function on ~2 hours sleep for days at a stretch.

    I do not consider it coincidence that around year 3 of this I started to develop some major health problems, including autoimmune and allergic reactions which I had never had before. I lack hard data, of course.

    Ironically, BP were busy manufacturing (or trying to manufacture) a “health and safety conscious, environmentally friendly” image of a socially responsible company at the time, to the point that they actually had people handing out red cards (no, seriously, they were, they did) to people who went up and down the stairs in the building without holding onto the handrails, because they might trip and hurt themselves. When I collated a stack of papers about the problems of shift patterns and sleep deprivation, and handed them to the health & safety officers at several levels up and down the HSE heirarchy, though, you can guess just how much attention got paid. Heck, you should have seen the shift patterns their Emergency Control and Response team got put on!

    Have I mentioned that I despise hypocrisy?

    I also despise shift patterns, and I find myself hoping that there actually IS a God — in order that there might be a hell, in order that the beancounters and so-called managers who devised those shift patterns find themselves there, sharing a circle with serial rapists.

  6. #6 kennykat
    July 28, 2008

    Despite the health risks to their employees, do you think that the accountants might consider diminishing returns?

    I’ve worked night shifts before and especially when trying to establish it as routine I experienced decreased working capacity. I can’t imagine the loss of effective man hours lost to a schedule like that. Let’s face it some hours of the day are more productive than others. Friday afternoon 4:30p.m. is more condusive to cleaning up your work area and tie-ing up important loose ends than actual production.

    Perhaps it is just that the cost aren’t as apparent as an employee and benefits. Or wrapped in a self-fullfilling prophecy. (If you don’t have the benefits, it’s hard to measure them)

    Perhaps if the accountants spent as much energy as they did designing your work schedule, they might be able to count the real cost :

    Higher turnover (statistical, exit interviewing), hiring, training cost, resulting overhead cost (total hours or increased number of salary positions), and safety, and other liabilities.

    Perhaps if they really understood this you’d start your shift together and do Tai Chi every morning.