How Your Body Tells Time

Your body always knows what time it is, even if you don’t. It may sound absurd to think that you don’t know what time it is. You are probably hyperaware of the time at every moment of the day.

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You have a train to catch or you have kids to drop off at school. You have a meeting in fifteen minutes and a call in an hour. You have to get to the dry cleaner before it closes. You have project deadlines, dinner reservations, and an alarm clock (or two) that wakes you every morning.

My clients tell me that they are constantly aware of the time and that the clock dictates nearly every one of their daily activities. But there is a different kind of clock inside your body, one that rules all of its cells and systems. To understand how it works, you have to step inside the brain and into the hypothalamus.

Hypothalamus

The hypothalamus sits at the center of the brain and is responsible for regulating all of the body’s systems.

It activates the fight-or-flight response when you feel stress or danger. It tells you when you are hungry or thirsty.

When you begin a strict diet, the hypothalamus is what’s telling you that you are starving because you are eating differently.

You may know that you aren’t starving, but the body is signaling to the brain that It’s not getting the same amount of food as before.

When you start a new exercise routine, the body signals muscle fatigue and cardiovascular stress to the brain and the hypothalamus urges you to stop.

And when you stay up late to work on a project, the hypothalamus is what’s telling you that you are sleepy and bored.

So this part of the brain can read the body’s signals and try to affect your behavior, trying to keep everything the same as it was yesterday.

 The hypothalamus also regulates all kinds of things that you don’t consciously control, including body temperature, hormone balance, and metabolism. All of these changes happen at predictable times of day.

For example, your body temperature peaks in the evening, then decreases during the night and reaches its lowest point just before dawn. Your blood pressure rises sharply as you wake up each morning, and then increases slowly throughout the day before falling during the night. The sharp rise of blood pressure in the morning comes at a time when blood platelets are stickiest, which explains why many heart attacks happen first thing in the morning. Cortisol levels, too, change at predictable times.

Cortisol

Cortisol is a steroid that the body produces and it’s sometimes called “the stress hormone.”

The level of cortisol in your body is lowest when you go to bed and then accumulates during the night.

It is partially responsible for your body’s inflammatory response, so it’s no wonder that those aches and pains are at their worst when you get out of bed, or that you feel most bloated and puffy in the morning.

Cortisol levels discharge steadily throughout the day, fluttering briefly upward after every meal.

Colonic Motility

Colonic motility—which is a fancy term for bowel movements—changes during the day as well.

First thing in the morning, the colon wakes and moves at three times its normal level of activity, with predictable results.

That’s why so many people feel constipated while in the throes of jet lag.

A poor eating schedule can also confuse the colon. At night, the colon rests and bowel movements are suppressed.

Mood and brain waves alter throughout the day and night as well. In order to regulate the body’s systems, your hypothalamus takes its cues both from the body’s tissues and organs and from the environment. When you smell food, you feel hungry; when you see danger, you feel anxious and energized for action.

Health & Spirituality

All true.

But let’s don’t forget the most pervasive signal the brain takes in all day—the presence of light. There is a small part of the hypothalamus, called the suprachiasmatic nucleus (SCN) that is tasked with noticing light.

It’s about the size of a grain of rice, and it contains approximately twenty thousand neurons.

Physiologists

Physiologists have long understood that these neurons respond to light and regulate the body’s systems based on light and darkness.

When light hits the retina of the eye first thing in the morning, the SCN signals to the body that it’s daytime.

In the evening, the SCN helps signal the body’s natural production of melatonin that tells you when it’s time for sleep.

But it’s only in the past twenty years that researchers have looked at how much power this tiny bundle of neurons exerts over every cell and system in the body.