Thursday, October 23, 2008

Power Napping

Sleep deprivation is chronic in our society and this has important implications for our health. Chronic sleep deprivation typically increases the production of proinflammatory cytokines. Chronic sleep deprivation will induce increased production of cortisol and interleukin 6. Both have a number of functions in physiology.

Cortisol is a key stress hormone that is important in suppressing excessive inflammatory responses and helping us to maintain alertness. Throughout the 24 hour period both cortisol and interleukin 6, along with a host of other physiological mediators, vary in accordance with our circadian rhythms. Cortisol rises in the early hours of the morning, helping us to wake up, climbs through the morning, then starts a gradual decline through the evening and night. At basal physiological levels it plays important roles.

Interleukin 6 is a perplexing cytokine, it has been associated both with inflammatory and growth processes, my guess is that its function is contingent on the expression of inflammatory mediators like tumour necrosis factor. That is, in the presence of tumour necrosis factor at elevated levels, in can promote inflammation. It is worth noting that in this study at least the levels of tumour necrosis factor were largely unaffected.

In this study the human subjects were sleep deprived for a number of days and then allowed to nap for 2 hours. The effects were minor, statistically significant yes but physiologically significant? Not so sure about that. Napping does help but it cannot make up for maintaining good sleeping patterns.

The Lesson

Don't fall for the idea that you can catch up on sleep and no harm done. Sleep deprivation is a stressor and will impact your performance. Chronic sleep deprivation will do you no good at all. It will do the exact opposite. During the deepest phases of our sleep cycle growth hormone levels are elevated, and various repair processes come into play. As high cortisol is often found in depression, chronic sleep deprivation can pave the way for depression. Indeed it can be the final straw that breaks the camel's back. So if you are experiencing chronic sleep deprivation try to find ways to change that.

I take good quality sleep so seriously that I wear a sleep mask and ear plugs. This has a wonderful effect on my sleep cycle and I typically wake up wonderfully refreshed. A great change from years gone by when I would wake up feeling like death. I am a night owl who likes to stay up late, typically reading, studying, or just playing computer games. However I do make sure I get a good night's sleep and this has substantially improved my productivity and cognition.

If you're interested in learning about sleep and the importance of maintaining regular sleeping patterns I highly recommend The Promise of Sleep by William C. Dement and Christopher Vaughan. Dr. Dement commenced the Stanford Sleep Centre and is a leading authority on sleep. The text is very accessible. Here is a link to it.

You can download the full paper here.

Am J Physiol Endocrinol Metab 292: E253–E261, 2007.
Daytime napping after a night of sleep loss decreases sleepiness,improves performance, and causes beneficial changes in cortisol and interleukin-6 secretion

A. N. Vgontzas,S. Pejovic, E. Zoumakis, H. M. Lin, E. O. Bixler, M. Basta, J. Fang, A. Sarrigiannidis, and G. P. Chrousos

Daytime napping after a
night of sleep loss decreases sleepiness, improves performance, and
causes beneficial changes in cortisol and interleukin-6 secretion. Am J
Physiol Endocrinol Metab 292: E253–E261, 2007. First published August
29, 2006; doi:10.1152/ajpendo.00651.2005.—Sleep loss has been
associated with increased sleepiness, decreased performance, elevations
in inflammatory cytokines, and insulin resistance. Daytime napping has
been promoted as a countermeasure to sleep loss. To assess the effects of
a 2-h midafternoon nap following a night of sleep loss on postnap
sleepiness, performance, cortisol, and IL-6, 41 young healthy individuals
(20 men, 21 women) participated in a 7-day sleep deprivation experiment
(4 consecutive nights followed by a night of sleep loss and 2 recovery
nights). One-half of the subjects were randomly assigned to take a
midafternoon nap (1400–1600) the day following the night of total sleep
loss. Serial 24-h blood sampling, multiple sleep latency test (MSLT),
subjective levels of sleepiness, and psychomotor vigilance task (PVT)
were completed on the fourth (predeprivation) and sixth days (postdeprivation).
During the nap, subjects had a significant drop in cortisol and
IL-6 levels (P < 0.05). After the nap they experienced significantly less
sleepiness (MSLT and subjective, P < 0.05) and a smaller improvement
on the PVT (P < 0.1). At that time, they had a significant transient
increase in their cortisol levels (P < 0.05). In contrast, the levels of IL-6
tended to remain decreased for ~8 h (P < 0.1). We conclude that a 2-h
midafternoon nap improves alertness, and to a lesser degree performance,
and reverses the effects of one night of sleep loss on cortisol and IL-6.
The redistribution of cortisol secretion and the prolonged suppression of
IL-6 secretion are beneficial, as they improve alertness and performance.

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