Not all time spent sleeping is the same. For the first five or 10 minutes after we doze off we are in stage one of sleep—the transition phase; people often insist they were not asleep when awakened during this stage. Stage two, which makes up about 50 percent of sleep time, is also light; the heart rate slows and body temperature drops, preparing the body for deep sleep. Stage three—deep sleep—is where much of the important work is done. It is divided into periods of rapid eye movement (REM) sleep and non-REM sleep. The brain is very active during REM sleep, and this is when dreams occur. Scientists aren’t sure why we dream, but they suspect it has something to do with helping us processes emotions. During non-REM sleep, the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system. Throughout our time asleep, we cycle through periods of light stage, REM, and non-REM sleep.
“Sleep needs vary with age,” says Ordovs. “We know that babies, young children, and adolescents need more sleep than older adults.” (See How Much is Enough? below)
While some sleep disorders have environmental or behavioral triggers, several genes have been identified that influence our circadian rhythms, the timing of sleep, and susceptibility to sleep disorders. It is important to seek diagnosis and treatment if you have trouble with sleep.
Sleep, Hunger, and Weight: Getting an adequate amount of quality sleep is an often-overlooked tool for helping maintain a healthy weight. “Reduced sleep has been shown to alter the hormones involved in hunger and satiety, along with subjective hunger and preferences for ‘comfort foods’ including highly processed junk and snack foods,” says Dariush Mozaffarian, MD, DrPH, dean of the Friedman School and editor-in-chief of Tufts Health & Nutrition Letter. In a prospective cohort study published in 2011 in the New England Journal of Medicine, Mozaffarian and colleagues looked at weight gain over 20 years in more than 120,000 men and women. “Our study found that, after adjusting for other factors, people who slept less than six hours per night had significantly greater weight gain than people sleeping six to eight hours per night.”
A 2015 study by Ordovs and colleagues found that longer habitual sleep was associated with lower body mass index (BMI). Several studies point to late-night snacking and less-healthy food choices as part of the problem. Research suggests that hormonal changes related to inadequate sleep time could increase the desire to munch: while not all studies agree, some early research found that levels of leptin (a hormone that helps us feel full) decreased with sleep deprivation, and levels of ghrelin (known as the “hunger hormone”) increased. Whatever the reason, lab studies that restricted participants to less than six hours of sleep found an increased intake of 300 to 550 calories a day, mostly from late-night snacking. This could translate to weight gain of around a pound a week. On the positive side, the study by Ordovs and colleagues mentioned above suggests that longer habitual sleep may help overcome some genetic variants that predispose people to obesity.
Sleep and Health: In addition to weight gain, short sleep duration has been associated with numerous health problems. Meta-analyses indicate a 30 percent increased risk of diabetes in people who sleep less than five to six hours a day, and in those who sleep more than eight hours a day. Several studies and meta-analyses have reported associations between short sleep duration and high blood pressure (but no such association has been reported with long sleep duration). Analysis of medical records of over 31,000 adults found that short sleep duration was independently associated with atrial fibrillation, an irregular heartbeat that increases risk of heart failure, kidney problems, stroke, coronary heart disease, and death.
A small recent study found that deep sleep helps relieve anxiety, and a sleepless night can raise subjective anxiety by up to 30 percent. Brain scans on 18 sleep-deprived young adults revealed deactivation of a brain area linked to calming anxiety and stress, and excessive brain activity in regions associated with processing emotions. After watching emotionally unsettling videos, participants reported much lower anxiety levels after a full night’s sleep than after a sleepless night, especially if they had gotten more deep, non-REM sleep.
A 2019 study by Ordovs and colleagues found that lower sleep times and fragmented sleep are independently associated with higher risk of plaque build-up in the arteries (atherosclerosis). The researchers performed sleep studies on nearly 4,000 participants. They also took three-dimensional ultrasounds and CT scans to look for build-up of plaque in the carotid and femoral arteries as well as calcification in the arteries that supply oxygen to the heart muscle. Lower sleep time and fragmented sleep were both associated with plaque build-up in all these areas. “Our research shows that sleep disruption, whether in quantity or quality, has long term, insidious, and dangerous consequences,” says Ordovs. “It promotes the silent buildup of plaques within our arteries, raising the risk of cardiovascular disease, the leading killer in our society.” Moreover, this happens above and beyond the relation of poor sleep with other cardiovascular risk factors such as obesity, diabetes, and hypertension.