Design for Life

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Have you ever noticed that some days you’re “on”—and some days you’re decidedly not? Are you more productive at some times than others? Do you push yourself to keep going when you’re tired? Do you ever just feel out of sync?

Before electric lighting dominated our world, we lived by the cycles of the sun and moon. We awakened with the sunrise, were active in the daylight, and gathered around the fire as darkness fell. This way of life wasn’t merely convenient; it turns out to be vital to our physical and mental functioning, our moods, and even our survival. In fact, many researchers believe that being synchronized with natural light-dark cycles is the basis of good health.

Living indoors, it’s easy to forget that our bodies have been intimately linked with natural cycles since life began. But it makes sense: As life evolved, the influence of the sun and moon were constant; organisms had to adapt to the cycles of day and night, a waxing and waning moon, and the seasons if they were to thrive.

In our own bodies, we have daily, monthly, and seasonal cycles of sleep and wakefulness, temperature, blood pressure, hormone secretion, cell division, and virtually all functions. These processes are triggered by changes in light level, air temperature, and other environmental factors.

Morning and evening are especially significant times for resetting our inner clocks. Awakening gradually with the sun, which stimulates the hormone serotonin, allows our body to peacefully resolve its sleep cycles and prepare us for the day. If we are in tune, our heart rate, blood pressure, body temperature, and cortisol (a hormone that defends against stress) level increase before we wake up. In the evening, these functions should decrease, while darkness triggers increased production of the sleep-inducing hormones melatonin and prolactin.

Both women and men experience monthly cycles in hormone production and other bodily processes. Many people believe that women’s menstrual cycles were once regulated by the cyclic changes in moonlight. Electric lighting, including streetlights, may throw off those cycles, resulting in menstrual irregularities, infertility, premenstrual depression, and difficult childbirth.

Our annual body cycles follow the seasons: As the days lengthen toward summer, our daily alertness phase also lengthens; our hearts and lungs handle exercise better; and we need less sleep. In winter, we become slower, we sleep longer and more deeply, we eat more, and we store body fat more easily; we move toward hibernation.