For some people, the slightest noise awakens them at night. For others, the wailing siren of a passing fire truck doesn't disturb their slumber. Just why, though, remains a bit of a mystery.
Although many people are self-proclaimed light sleepers or heavy sleepers, researchers have found that little is actually known about why people react differently to noises and other stimuli during sleep.
Genetics, lifestyle choices, and undiagnosed sleep disorders may all play a role. In addition, some studies suggest that differences in brainwave activity during sleep may also make someone a light or heavy sleeper.
Light And Deep Sleep
During sleep, you alternate between cycles of REM (rapid eye movement) and NREM (non-rapid eye movement) that repeat about every 90 minutes. You spend about 75 percent of the night in NREM sleep, which consists of four stages of increasing relaxation.
Stage one, or the phase between being awake and asleep, is considered light sleep. Deeper sleep begins in stage two, as your breathing and heart rate become regular and your body temperature drops.
Stages three and four are the deepest and most restorative stages of sleep, in which breathing slows, muscles relax, and tissue growth and repair occurs.
In general, young people spend more time in the deeper, heavier stages of sleep as they grow and develop. Older people spend less time in deep-sleep stages and are more likely to complain of being light sleepers.
But sleep experts say the difference between a light and heavy sleeper may be largely subjective. Someone who gets eight hours of sleep a night may not experience as much slow-wave, deep sleep as the person who get six hours of sleep.
"There may be some overlap between what people subjectively feel about the depth of their sleep, and what we find in the lab in measuring the different sleep stages," said David Neubauer, M.D., an assistant professor of psychiatry and behavioral sciences and associate director of Johns Hopkins Sleep Disorders Center in Baltimore, Md. "But it's not necessarily the same thing."
What Contributes To Light Sleep
A small study, published in 2010 in Current Biology, suggests that differences in how sleeping people respond to noise may be related to levels of brain activity called sleep spindles. The researchers found that people whose brains produced the most of these high-frequency sleep spindles were more likely to sleep through loud noises. But more research is needed to confirm the results.
Neubauer said that if someone is complaining of not feeling rested because of being a light sleeper, they should look at the factors that might be contributing to the inability to achieve a deep sleep.
A doctor can recommend a sleep study in a sleep lab to see if a sleep disorder may be to blame.
Some sleep disorders, such as obstructive sleep apnea, may contribute to light sleep by causing awakenings throughout the night because of breathing irregularities.
Neubauer added that it's hard to generalize about what makes some people light sleepers and others heavy sleepers. "It might be some sort of genetics, or it might be that some people have a greater degree of arousal over a 24-hour cycle," he said.
In most cases, however, factors under your own control affect the quality of sleep you get. "There are lots of issues related to lifestyle, medication, alcohol, and caffeine that can lighten sleep," Neubauer said. "People might also not be getting enough sleep because they're not spending enough time in bed due to the choices they make."
Practicing healthy sleep habits -- maintaining a regular sleep schedule; limiting caffeine and alcohol use; and sleeping in a quiet, dark, and cool space -- can all help foster deeper, heavier sleep.
"Light Sleepers vs. Heavy Sleepers" originally appeared on Everyday Health