(dailyRx News) So you figure you got 8 to 9 hours of sleep, so you're good - even if it's spread out a bit with a nap, right? Wrong. Because of your circadian rhythms, all sleep may not be equal.
Your circadian rhythms are biological cycles that help determine your sleep/wake cycles, among other things.
Disrupting them can have health consequences, and they are generally aligned to the cycles of day and night.
A recent study has found that not getting enough sleep at night - in the dark - can throw off your circadian rhythms, which has a ripple effect into other areas of your health.
The biggest culprit? Keeping the lights on into the night.
The study, by Helen J. Burgess at the Biological Rhythms Research Laboratory in the Department of Behavioral Sciences at Rush University Medical Center in Chicago, aimed to look at the impact of ambient light used in evening on people's circadian rhythms.
Twelve people went through a two-part experiment that altered how much sleep they got before seeing the bright light of morning.
The participants, with an average age of 28, were nonsmokers who were a healthy weight, not taking any medications and drank only a moderate amount of caffeine (less than 300mg daily) and alcohol (less than two drinks a day).
They also reported having no medical, psychiatric or sleep disorders in pre-study questionnaires, and none were colorblind (which might affect their reaction to the light in the study). They also were not night-shift workers and had not crossed more than one time zone in the month before the study.
Over the 48-day study, the participants had approximately one-week long periods to normalize their sleep and were then exposed to two different scenarios.
In one scenario, the participants had a nine-hour window to sleep before morning bright light. In the other, they were provided time for a three-hour nap during the daytime and then a six-hour nighttime window for sleep before morning bright light.
Eight of the 12 participants were able to get the same amount of sleep in both scenarios, but four participants lost about 40 minutes of sleep or more during the short nights.
Overall, the whole group got much less sleep - an average 31 hours less over the study - in the nap-plus-short-sleep scenarios than in the long-sleep scenarios.
While the eight who got equal amounts of sleep in both scenarios did not experience any more sleepiness in one scenario or another, they did experience differences in their circadian rhythm phases.
Basically, their phase did not "advance" as much in the short nights as it did during the long nights leading up to when they were exposed to the bright morning light. This means their circadian rhythms might be more likely to get off-kilter, even if they appeared to get enough sleep.
"By reducing phase advances to morning light, short sleep episodes may increase the risk for circadian misalignment and the associated negative impact on health, mood, sleepiness and cognitive performance," the authors wrote in their background information.
The authors said that the "most likely causes" of the reduced phase advance - the mechanism that could misalign their circadian rhythm - was the extra three hours of ambient light on the short sleep nights.
"Indeed, this additional evening light in the short nights condition may have reduced the sensitivity of the circadian system to the bright light the next morning, thereby reducing subsequent phase advances," they wrote. "The evening ambient light may have also phase-delayed the circadian clock, as repeated exposure to dim ambient light in the home environment has been shown to delay circadian phase."
As William Kohler, MD, the director of the Florida Sleep Institute in Spring Hill, Florida, explained, humans' body clocks are on an approximately 24.2 to 24.3-hour cycle, just slightly off from the 24-hour cycle of a day. Exposure to the bright light of morning helps reset those circadian clocks to re-align with the 24-hour cycle.
However, if a person is exposed to artificial light in the evening, whether from electronics or lamps, it can lessen the re-setting effect. A person with a normal bed time of 10 P.M. who was exposed to too much ambient artificial light in the previous night may not feel sleepy until 10:15 P.M.
Then, if they had the light on right up until they went to bed at 10:15 P.M., then they may not feel sleepy the following night until 10:30 P.M. because the morning light doesn't reset their clock as effectively.
"Light has an enormous effect on our circadian clock and the timing of that light is crucial," said Dr. Kohler, who was not associated with this study. "It's well known that light in the evening can further delay our clock, and bright light in the morning at the proper time will help advance the clock. If we had ambient light in the evening, the clock would not have advanced perhaps as much to get us ready to go to bed when we want to."
So checking your email on your phone while laying in bed, for example, could be having a bigger impact on your sleep schedule than you realize.
"This is an important piece of research because it further demonstrates how light affects our ability to go to sleep," Dr. Kohler said. "The effect of light in the evening from the house, computer lights, the TV, etc. can affect the ability of that light in the morning to change that clock. The reduction in phase advance means the bright light in the morning doesn't have as strong an effect in advancing the clock."
Up to 27 percent of Americans report getting less than six hours of sleep on weeknights, according to the National Sleep Foundation, primarily because of later bedtimes. The combination of a later bedtime with having on lights in the evening may make it tougher to get up for work in the morning or deal with other schedule issues.
"Thus, short sleepers are likely to have more difficulty adjusting to eastward jet travel, early morning shift work and even regular work hours after a late weekend, as adjustment to these situations requires circadian phase advances," the authors of the study wrote.
The take-away is that too much ambient light in the evenings might disrupt a person's circadian rhythms over time, especially if they do not get enough sleep.
The study was published August 13 in the Journal of Sleep Research. The research was funded by grants from the National Heart, Lung and Blood Institute at the National Institutes of Health. The authors reported no conflicts of interest.