Sleep is an alternate cycle of REM (rapid eye movement) and NREM (non-rapid eye movement) that repeat about every 90 minutes. We spend about 75 % of the night in NREM sleep. It consists of four stages of increasing relaxation. Stage one is the phase between being awake and asleep and is considered as light sleep. Deeper sleep begins in stage two, as our breathing and heart rate become regular and the body temperature drops. Stages three and four are the deepest and most restorative stages of sleep. In these stages breathing slows and muscles relax, and tissue growth and repair occurs.
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.
Some studies suggest that differences in brainwave activity during sleep may also make someone a light or heavy sleeper. Genetics, lifestyle choices, and undiagnosed sleep disorders play a role.
According to research published in 2010 in Current Biology, how we respond to noise while sleeping may be related to levels of brain activity called sleep spindles. People with brains producing the most of these high-frequency sleep spindles are more likely to sleep through loud noises.
Sleep disorders like obstructive sleep apnea may contribute to light sleep by causing awakenings throughout the night because of breathing irregularities.
In most of the cases, factors under our own control affect the quality of sleep we get. Issues related to lifestyle, medication, alcohol, and caffeine can lighten our sleep. Practicing healthy sleep habits,maintaining a regular sleep schedule,limiting caffeine and alcohol use and sleeping in a quiet, dark, and cool space can help foster deeper and heavier sleep.