Unit 1 Taking Charge of Your Health and Wellness Copyright Goodheart-Willcox Co., Inc. 70 A person’s sleep-wake cycle is his or her pattern of sleeping in a 24-hour period. The biological clock regulates the sleep-wake cycle in two ways. First, it monitors the amount of light in the environment. If it senses light, the biological clock sends signals in the body that result in activity. If there is less light, the biological clock can send signals to make the body less active. The biological clock also causes a gland located in the brain to release the hormone melatonin when it gets dark (Figure 3.5). Melatonin increases feelings of relaxation and sleepiness and signals that it is time to go to sleep. When the natural circadian rhythm is disrupted, the body’s biological clock takes a while to readjust. This explains jet lag, which is a fatigue that people feel after changing time zones when they travel. When people travel by plane from California to New York, their bodies feel like they “lost” three hours. When their alarms ring the next morning at 7 a.m., they are tired because it is only 4 a.m. according to their biological clocks. Jet lag, however, is not the only possible disruption of the sleep-wake cycle. Working night shifts, adjusting the clock for daylight savings, or simply using electric lights late into the night can trick your body into an unnatural circadian rhythm. Stages of Sleep Each night, you usually pass through five distinct stages of sleep (Figure 3.6). A complete sleep cycle—from Stage 1 through Stage 5—lasts about 90 to 110 minutes. Over the course of a night, you go through this sleep cycle three to five times, depending on how long you sleep. The amount of time you spend in each stage of sleep, however, changes considerably as the night progresses. Circadian Rhythms in Humans 12:00 Midnight 12:00 Noon 6:00 a.m. 6:00 p.m. 2:00 a.m. Deepest sleep 4:30 a.m. Lowest body temperature 6:45 a.m. Sharpest rise in blood pressure 7:30 a.m. Melatonin secretion stops 10:00 a.m. High alertness 3:30 p.m. Fastest reaction time 2:30 p.m. Best coordination 5:00 p.m. Greatest heart efficiency and muscle strength 6:30 p.m. Highest blood pressure 7:00 p.m. Highest body temperature 9:00 p.m. Melatonin secretion starts Clockwise from top: kdshutterman/Shutterstock.com Sanit Fuangnakhon/Shutterstock.com estherpoon/Shutterstock.com sihy/Shutterstock.com Figure 3.4 The body automatically adjusts to specific times, providing better coordination and alertness during the day and deepest sleep at night. When does the body start secreting melatonin to help encourage sleep? My Life Graphic/Shutterstock.com Figure 3.5 A person’s body will only produce melatonin when the person is in an environment that is dark or dimly lit. Sunlight and bright indoor lighting can prevent the release of melatonin. What is the body’s physical reaction to the hormone melatonin?