The time you go to sleep at night and the time you wake up in the morning can be crucial to your health—and not only because you need enough sleep. It’s also because you need to sleep at the right time of the night. For good health, you need to match your bedtime and wake-up times to your personal “clock genes”—genes that control your sleep/wake cycle along with many other biological fluctuations such as blood pressure and blood sugar. If you don’t, circadian disruption can result, putting your health in danger…


Your biological rhythms are timed to Earth’s 24-hour ­cycle of light and dark. But your body also runs on internally generated cycles that are a nearly perfect match for the 24-hour rhythm of day and night. Called circadian rhythms, these internal cycles are governed by the suprachiasmatic nucleus (SCN), a “body clock” located in the hypothalamus ­region of the brain.

Responding to light and dark, the SCN sends signals to every part of your body, regulating when you fall asleep and wake up, when you’re hungry, when blood sugar levels rise and fall, when hormones are released—in fact, nearly every biological and behavioral process is regulated by the SCN.

In addition, there are clock genes that produce proteins that regulate rhythms within cells—scientists estimate that 10% of all genes are involved in the body’s rhythms. Clock genes help determine whether you tend to go to bed early and wake up early…or go to bed late and wake up late.

When the external rhythms of light and dark don’t match your body’s unique circadian rhythms—a condition called circadian disruption—your health suffers. Circadian disruption has been linked to many health problems (see below).

Fortunately, there are ways to prevent or reverse circadian disruption, including the following…

Match your rhythm to your sleep schedule. Some people have clock genes that run slightly faster than the 24-hour ­day-night cycle, making them tired earlier in the evening. Some people have clock genes that run slower, making them tired later in the evening. Which are you?

How to tell: If you usually feel at your best—your most awake and alert—in the morning, you’re what I call a “morning type.” If you usually feel your best in the evening, you’re an “evening type.”

To be healthy, you must synchronize these genetic rhythms with your bedtime and wake-up time. In addition to timing, stability of sleep and wake times is important. When there’s more than a two-hour difference between the time that is best for you to go to bed and when you actually go to bed, you’re desynchronized—and your health is at risk.

What to do: Establish a bedtime and wake-up time that reflect whether you’re a morning type (Example: 10:30 pm bedtime and 6:30 am wake-up time) or an evening type (Example: Midnight and 8:00 am). Don’t vary those times by more than one-half to one hour. If you work full-time during the week, don’t vary those times by more than two hours during the weekend.

Get more light. One study compared people who worked near a window to windowless workers and found that those next to windows had better overall health and slept an average of 46 minutes more per night during the workweek over a one-month period—because daily exposure to light helped prevent desynchronization.

What to do: Try to get natural light during the day. Even 15 or 20 minutes a day can help.

Get less light in the evening. While bright light in the morning and afternoon is synchronizing, bright light in the evening is desynchronizing—­because it blocks the production of melatonin, a hormone that helps regulate the sleep-wake cycle.

Recent research: Exposure to artificial light at night was linked to a 52% increased risk for breast cancer. But reducing artificial light at night—by using a bedtime reading lamp instead of overhead lights, for example—decreased risk by 81%. And a study published in International Journal of Obesity found that exposure to artificial light at night was linked to a 97% increased risk for obesity.

What to do: Two to three hours before bedtime, turn off overhead lights to reduce brightness. Use table lamps instead.

At night, avoid the melatonin-­suppressing “blue light” of electronic devices, such as computers, tablets and cell phones. LED TV screens give off blue light, too. If you must use blue-light devices at night, wear blue-­blocking glasses or goggles, widely available online for less than $10. Or consider downloading an app such as f.lux or Twilight, both of which reduce blue light on your screens at night.

Eat regular meals. A regular eating cycle is as important as a regular sleep-wake cycle in keeping you synchronized. Morning types tend to eat a big breakfast and smaller dinner. Evening types tend to favor dinner and skip breakfast.

What to do: Whatever your type, try to eat your three meals at about the same time every day.

Finish dinner at least four hours before bedtime. That habit not only helps you stay synchronized but also helps prevent weight gain—studies show the closer to bedtime you eat, the more likely you are to be overweight.

Structure your day. In addition to regular mealtimes, regular times for socializing, exercising and other daily activities help prevent desynchronization, improving sleep and memory—particularly in people age 55 and older.

What to do: Exercise at the same time, several days a week. Get out of the house, and engage in regular, ­structured social activities such as going to the library or a place of worship. It’s the day-to-day regularity of a structured schedule—doing one or more activities the same time every day, a minimum of three to four days a week—that helps create synchronization.

Consider taking melatonin. Taking a melatonin supplement in the right dose at the right time can help restore synchronization and may benefit those evening types who can’t fall asleep at bedtime.

What to do: Take one-half to one milligram of melatonin five to six hours before your desired bedtime—that’s the right amount and timing to maximize melatonin levels at bedtime.

Dangers of Circadian Disruption

Circadian disruption has been linked to many health ­problems, including…

  • Insomnia
  • High blood pressure, heart attack and stroke
  • Obesity and diabetes
  • Memory problems, dementia and Parkinson’s disease
  • Depression and bipolar disorder
  • Heartburn, IBS and other digestive diseases
  • Asthma, emphysema and chronic bronchitis
  • Poor recovery from surgery
  • Weakened immune system
  • Cancer