Nothing can boost energy–and brainpower–like a midday snooze. Behold, the restorative effects of a 30-minute slumber: As you nod off…Drowsiness is brought on by a sleep-promoting chemical called adenosine, which builds up in your body throughout the day. If you skip shut-eye at night, a high level of adenosine can leave you feeling desperate for a nap. Your brain pumps out GABA, a neurotransmitter that lets your head’s sleep-wake center know it’s sleepy time. GABA also helps deactivate much of the brain stem, which controls muscle movement. That’s why when you dream about, say, playing tennis, you don’t swing your arm. If you’re super-exhausted, your brain might shut down before your body is fully relaxed, leading to involuntary muscle contractions (those arm or leg jerks). Then, in the first five minutes…Your brain isn’t conscious, but your senses are still online. A sharp noise or a poke would rouse you. As your blood pressure and heart rate slow down, your eyes stop moving behind your lids and will remain sluggish for the rest of your snooze. In the next 20 minutes that follow…The accumulation of adenosine in your body breaks down. At the same time, your adrenal glands are readying a stash of cortisol to help you feel more alert when you wake up. Your immune system -thrown out of whack by fatigue -begins to reset itself back to normal. And finally, in the last five minutes… Most naps don’t go into deeper stages of sleep, but if you’re seriously drained, you might enter rapid eye movement (REM). Snapping out of REM is harder and can leave you groggy. If you arise from a 30-minute nap and still feel wiped out, you need solid Z’s. Catnaps can’t fix real sleep deprivation. Beep-beep-beep! Your alarm blares and, in a split second, your brain releases a torrent of chemicals that turn off its sleep center and jump-start your body. Your rested, awakened brain cells are now more capable and active. And, thanks partly to low adenosine levels, you feel refreshed. Sources: Alon Avidan, M.D. et al, associate director, UCLA Sleep Disorders Center;