Human sleep-wake cycles are set mostly by the sun. Light levels filtered through the retina send messages to the pineal gland, which pumps out or dials back on melatonin, the hormone associated with sleep regulation. Without sufficient exposure, melatonin levels are disordered, and our body’s innate rhythms are thrown out of whack. Disruptions in circadian rhythm have been linked to cancer, diabetes, obesity, and, in the short term, declining athletic performance and mental fog and gloom.

Natural light “of sufficient intensity and suitable spectral composition” is the main factor in regulating our circadian rhythms, according to a 2012 study of workers at the North and South poles. During dark polar winters, researchers found, a one-hour pulse of bright artificial sunlight in the morning was enough to stabilize workers’ circadian rhythms.

But we need darkness, too. Too much light, particularly at night, throws off our internal clock, as a series of recent experiments at Rensselaer Polytechnic Institute in Troy, New York, make clear. In the study, college students used backlit tablets for two hours before bed. Afterward, their melatonin concentrations dropped 23 percent.

DO: Soak up the sun. Those rays enable your body to synthesize vitamin D, a nutrient many of us are deficient in. A 2009 study of distance runners found that 40 percent had suboptimal levels of vitamin D. But a mere five to 30 minutes a day of ultraviolet radiation is sufficient for vitamin D synthesis, according to the study’s author, University of Wyoming associate professor D. Enette Larson-Meyer.

DON’T: Bring your iPad to bed. Mariana G. Figueiro, director of the Lighting Research Center at Rensselaer, recommends “reducing use of self-luminous displays at least two hours prior to bedtime.”