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Your cup of coffee may have met its match: Caffeine loses its effectiveness when you are continually short on sleep, a new study presented at a conference of the American Academy of Sleep Medicine and the Sleep Research Society suggests.
In the study, researchers restricted two groups of participants to 5 hours of sleep each night for 5 days. One group received 200 milligrams (mg) of caffeine—about what you’d find in a short brewed coffee at Starbucks—twice a day, while the other group took a placebo pill.
During the first two nights of short sleep, the people who received the caffeine were more alert, were in a better mood, and performed better on physical response tests than the placebo poppers.
But by the third night, the caffeine group experienced no significant difference in alertness and performance from the placebo group, and even reported feeling more annoyed, frustrated, and jittery.
The researchers aren’t exactly sure why the effects of caffeine are blunted after you’re short on sleep for a few days.
It may have something to do with how caffeine works in the first place, they believe.
When caffeine binds to the receptors in your brain for a sleep-inducing chemical called adenosine, it makes you feel more alert.
But when you’re continually sleep deprived, your body produces more adenosine.
And it’s possible that these extra molecules might be pushing the caffeine away from the receptors, preventing their stimulating effects, says lead study author Tracy Jill Doty, Ph.D., research scientist at Walter Reed Army Institute of Research.
More research needs to be done to determine whether increasing the caffeine dosage can help, she says.
But if you bump it up too far—once you go above 400 mg a day—you increase your chances of feeling negative side effects from it, like irritability and restlessness.
Your best bet is to give in to your body’s need for shuteye, and let yourself snooze for 7 to 9 hours a night to recuperate, says Tom Balkin, Ph.D., former chairman of the National Sleep Foundation Board.
If you can’t squeeze in those extra few hours, a short nap can help with the drowsiness and decline in alertness that comes with lack of sleep.
The National Sleep Foundation recommends 20 to 30 minute naps for a short boost in energy and performance. Anything longer than that can cause sleep inertia—your body’s physical transition period into waking up, which often leaves you feeling groggy.
Just try to squeeze it in midafternoon, like right after lunch, since taking a nap too early or too late in the day can make it hard to fall asleep at night.