Simply taking a couple of naps may counteract the impact of a sleep-restricted night on stress and immune markers, a finding that could potentially benefit night and shift workers or other chronically sleep-deprived populations, the results of a French study indicate.

Brice Faraut, PhD, from the Université Paris Descartes-Sorbonne Paris Cité, France, and colleagues found that, after a night with only 2 hours of sleep, taking two naps of just 30 minutes each appeared to normalize levels of cytokines and catecholamines.

"Napping as a countermeasure to sleep restriction could, in addition to benefits on alertness, improve neuroendocrine stress and immune recovery, with a potential prophylactic long-term effect on cardiovascular health," the researchers write.

The study was published online February 10 in the Journal of Clinical Endocrinology and Metabolism.

Dr Faraut explained to Medscape Medical News why it was important to undertake this study: "Increasing numbers of people are becoming chronically sleep deprived because of greater work pressure in urban economies, eg, extended working hours outside the regular [8:00 am to 5:00 pm] working day, shift work, or increased accessibility to media of all sorts."

"For instance, the 2009 National Sleep Foundation survey reported that the percentage of the population sleeping less than 6 hours per night on weekdays has almost doubled over the past 10 years, increasing from 12% in 1998 to 20% in 2009," he added.

Surprisingly Strong Data Need to Be Replicated

Despite the well-known beneficial effect of napping on alertness, its effects on neuroendocrine stress and immune responses after sleep restriction are largely unknown, Dr Faraut and colleagues explain in their paper.

They conducted their crossover, randomized study in which 11 healthy men aged between 25 and 32 years underwent two sessions of laboratory sleep testing in which their sleep–wake status, light environment, and caloric intake were strictly controlled.

In a "sleep-restriction" session, the participants slept for just 2 hours for one night after a baseline night of 8 hours of sleep, followed by a recovery night of sleeping. In the "sleep-restriction/nap" session, they repeated the protocol, but with two 30-minute naps after the sleep-restricted night.

Salivary samples were taken every 2 hours and analyzed for interleukin (IL)-6 levels, an inflammatory cytokine known to have diurnal variations in concentration. In addition, urine samples were taken every 3 hours to measure levels of epinephrine, norepinephrine, and dopamine.

In the sleep-restricted session, there was a significant 2.5-fold increase in norepinephrine levels in the afternoon compared with the control day (P = .003). However, those differences were not seen when participants were able to take two 30-minute naps.

No significant differences were seen either for epinephrine or dopamine, the researchers report.

After a sleep-restricted night, IL-6 levels were significantly lower at 10:00 am and 7:00 pm than levels measured at the same times after the control night's sleep (P = .01 and P = .05, respectively). Again, those differences were not observed when participants were able to take the two 30-minute naps.

During the recovery day after the sessions, the team also found that there was significantly increased release of epinephrine in the afternoon and early morning among participants who had undergone sleep restriction compared with the control day (P = .02 for both). This was not observed in those who had been allowed two 30-minute naps.

Interestingly, daytime napping was associated with a significantly reduced amount of slow-wave sleep during the recovery night compared with sleep restriction alone (P = .01) and a trend toward decreased total sleep time.

"Our data suggest that napping has stress-releasing and immune effects," the researchers conclude.

Discussing the findings, Dr Faraut explained that the ability of two 30-minute naps to essentially normalize IL-6 and catecholamine levels had not been expected.

"It was quite surprising to observe the strength of the data, but it was performed in young healthy subjects with a very efficient capacity of sleep homeostatic pressure recovery."

"It would need to be replicated in a larger population sample at different ages, and in chronic sleep-restricted subjects such as in night and shift workers," he pointed out, adding: "Night and shift workers are at higher risk than day workers for cardiovascular risk."

"Short nap duration (30 min in our study) could be a natural [and] powerful countermeasure to sleep debt from the perspective of extended working-hour conditions and night and shift work and could also have a protective effect against cardiovascular pathogenesis," he concluded.

Source: MedScape: Napping Restores Immune System After Sleep Deprivation. http://www.medscape.com/viewarticle/840175