Your Biological Clock - The Influence Of Your Biological Rhythms: Chronic Fatigue

Content provided by the Faculty of the Harvard Medical School

Your biological clock

To a large extent, your level of alertness (or its opposite, sleepiness) is dictated by your biological clock, a cluster of cells deep within the brain. These cells, called the suprachiasmatic nucleus, are in the hypothalamus, the brain center that regulates alertness, appetite, body temperature, and other biological states. Your biological clock, or circadian cycle, controls the rise and fall of hormones and other chemicals in your body that influence whether you feel awake or sleepy. Like the other clocks in your life, the biological clock operates on a 24-hour schedule.

According to this schedule, people typically feel most energetic in the morning until early afternoon, when their energy flags after lunch (see Figure 2). Many people get a second wind later in the afternoon, but their energy level dips again when the sun goes down, and it doesn't rebound until the next morning. In short, the biological clock helps explain why people wake up in the morning and go to sleep at night, and why their energy dips after lunch and falls further still after dinner.

Figure 2: When are you at your best? Scientists have measured several types of performance, including mental alertness and short-term memory, throughout the day. These skills usually peak in late morning. Lowest performance levels are usually in the late evening. The best-known temporary decline in performance is in the early afternoon and is known as the "post-lunch dip."

Not everyone follows this pattern. Some people are "night owls" who get a second wind in the evening and have trouble getting going early in the morning. But if you are more typical, it's no coincidence that your energy level rises and falls with the sun. Although it's largely self-regulating, your biological clock responds to daylight as an important clue to external time. Nerve fibers connect the hypo­thalamus to the retina in the eye. In experiments, when people live for several days in a windowless laboratory and are exposed to light at intervals that are at odds with the outside world, their biological clocks reset themselves to match the new pattern of light and darkness. The influence of light on sleep/wake patterns probably explains why as many as 90% of blind people have problems sleeping.

Your biological clock needs to reset itself when you fly to a different time zone. But the resetting takes time (see Figure 3). Until you become accustomed to the new time zone, you may suffer from jet lag, feeling tired when you need to be awake and wide awake when you should be sleeping. Fortunately you can take steps to reduce the impact of jet lag (see "Ways to minimize jet lag," below).

Figure 3: Reset your biological clock Traveling west to east Help reset your biological clock when you travel through time zones. If you'll be traveling through several time zones, as when flying from the west coast to the east coast, you can gradually adjust your sleep time. For example, three days before you plan to travel from the west coast to the east coast, go to bed half an hour earlier than usual, and get up half an hour earlier the next morning. The next night, go to bed 60 minutes earlier than usual and get up 60 minutes earlier. The day before you travel, make it 90 minutes. By the fourth day, the day of your trip, you'll find it easier to adjust to your new time zone.

Another challenge to the body's biological clock occurs in people who work the night shift and sleep during the day. The biological clock can adapt to this reversal of normal wakefulness and sleep, but only if the person sleeps in a very dark room. And just as with changing time zones, the change in your body's clock does not occur instantly, but rather over several days. In the meantime, you're likely to be less alert and to function less well than usual.

Last updated:	January 23, 2007