Enter the Chronotherapists
By OLIVIA JUDSON
Olivia Judson on the influence of science and biology on modern life.
Here’s my prediction for the Next Big Thing in health care: chronotherapy, or therapy by the clock. Yes, in the future, your medicines, your operations, your mealtimes and when you step onto the treadmill or the badminton court — all will be overseen by your personal chronoconsultant.
It’s been known for ages that our bodies have daily, or “circadian,” rhythms. Body temperature is lower in the morning than it is in the afternoon. Blood pressure is low during the night, and rises just before you wake. Muscles are stronger in the afternoon than they are in the morning, and you may have greater dexterity then, too. Badminton players tend to serve more accurately in the afternoon, for example.
But now it’s clear that the body clock is in charge of many other, more subtle processes as well. The content of human breast milk changes during the day. Evening milk is full of compounds that make a baby sleepy; morning milk isn’t. The liver, too, has a strong daily rhythm: many of its activities shut down during the night. Levels of several hormones, including melatonin (involved in sleep) and ghrelin (involved in appetite), rise at night. Testosterone, in contrast, is highest in the morning and lowest in the late afternoon. Cholesterol is made more rapidly at night. Even cancers have a rhythm: breast cancers, for instance, grow faster during the day.
The implications of all this are huge. Living against your body clock — as so many of us do — can affect your health and well-being in myriad ways. Some of these are trivial: unless you’re professional (or super-competitive), it probably doesn’t matter if your badminton serve is a little off in your morning games. Besides, your opponent’s will be, too. (It may, however, be better for your heart if you play in the afternoon.)
But living against the clock can also lead to major health problems. Obesity, breast cancer and certain kinds of mental illness are all associated with circadian disruption.
Disruption can be a consequence of shift work or jetlag — or of not spending enough time sleeping, or in the dark. Darkness is important because even a brief exposure to light during sleep-time can be enough to reduce melatonin levels and reset the body clock. Exposure to light in the night has been linked to breast cancer; consistent with this, women who are totally blind have a lower incidence of breast cancer than those who can see even a little bit.
Badly timed light isn’t the only troublemaker. Eating at the wrong moments — like the middle of the night — makes it harder for the body to process food and leads to weight gain. A recent experiment shows this nicely. Two groups of mice were fed identical diets but on different schedules: one group was allowed to eat only during normal waking hours, while the other was restricted to eating during normal sleeping hours. After six weeks, the mice allowed to eat only during sleep-time were significantly fatter than the wake-time eaters — a result that may help explain why obesity is so common among shift-workers who, because of their jobs, are forced to eat against their clocks.