The human body is an incredible machine. It functions without having to consciously maintain itself, other than eating, drinking fluids and going to the bathroom.

Throughout the day, your body constantly changes, adjusting to its environment and the stress placed on it. You may never notice its changes and adaptations, but they occur every single day.

“I don’t think people realize how much their bodies are in flux throughout the day,” says Dr. Rocco Monto, a board certified orthopedic surgeon with a practice in Nantucket, Massachusetts.

Below, Monto explains six ways your body changes throughout the day.

1. You Get Shorter

Yes, that’s right. You actually shrink throughout the day. And you can blame gravity.

Dr. Monto explains that the disks in your spine are gelatinous. When you stand or sit, gravity compresses your disks, pulling you down. Over the course of the day—and especially if you’re dehydrated—your discs compress enough to cause you to lose some height. Monto says, “You have about a one percent height loss, which you get back at night when you lie down and hydrate.”

You might experience an even greater height loss if you participate in a ballistic activity, such as running.

Incremental height loss is not necessarily an issue—that is, unless you’re concerned about an upcoming height test. Remember, you get back to your normal height at night, when you stretch out between the sheets.

2. You Gain Weight

If you step on the scale several times during the day, you’ll never see the same number. What gives?

Obviously, you ate food and drank fluid, which contribute to weight fluctuations. If you did a workout and were super sweaty, you might actually lose weight from the fluid loss. Side note: Make sure to rehydrate.

But other things contribute to weight changes, besides what you put into your body and what leaves it.

According to Dr. Monto, the body is programmed to gain a little weight in the afternoon due to cortisol fluctuations. Cortisol is a stress hormone that regulates protein, carbohydrate and fat metabolism, among other things. Cortisol levels peak in the morning, signaling your body to create energy for the day ahead. Toward the evening, cortisol levels dip, slowing down your metabolism and allowing your body to store energy.

“Your weight gain occurs mostly as you head into the evening hours, because your body slows down a little bit,” says Monto. “Your body gets more efficient as you head into nighttime.”

RELATED: Why Cortisol Inhibits Workout Gains

3. Your Hormones Fluctuate

The body is a smart machine—so smart that it knows what you’re about to do before you do it. For example, it releases hormones based on your normal sleep/wake patterns. Basically, your daily habits program your hormonal cycles.

Dr. Monto explains that testosterone—responsible for muscle growth—maxes out at around 8:00 a.m. Serotonin levels, which control your alertness and muscle contractions, peak at midday.

“At midday, you’re at your highest level of alertness,” he says, “So mid-morning should be the target time for a lot of your activity if you have the choice.”

Testosterone and serotonin level off in the evening, and that’s when melatonin kicks in, preparing your body for sleep.

4. Your Heart Rate Changes

There are times during the day when your heart rate is higher or lower. There are also times when your heart rate can change rapidly, meaning that your heart is primed to work. “The quicker your heart rate response, the better time it is to train,” says Dr. Monto.

Training during your ideal heart rate response time can help you get more out of your workouts. You’ll be able to push harder and recover faster after a tough set. Does this mean you should work out only during this time? Absolutely not. It’s probably not a dramatic difference.

Dr. Monto says that ideal heart rate response times are highly individual. The way to find your ideal time is to track how your heart rate responds to exercise at different times of the day.

RELATED: Improve Your Endurance and Conditioning With a Heart Rate Monitor

5. You’re More or Less Likely to Get Hurt

Due to the hormone fluctuations described above, the body is better able to handle stress in the morning than in the evening. That’s why there’s a slightly greater risk of injury in the evening—when many practices and games take place.

Dr. Monto explains: “It’s probably related to a dipping of some of the stress hormones. Your body is a little less able to handle stress, and is more likely to sustain an injury.”

Sports are stressful—not like when you cram for an exam, but athletic activity places extreme stress on the body, pushing your physical limits. If your hormones are not in an optimal state to handle the stress, something might have to give—whether your performance suffers or you suffer an injury. Dr. Monto adds, “There are trends showing that there are more injuries in night games compared to day games.”

RELATED: 5 Things You Can Do To Prevent Muscle Injuries

6. Your Body Temp Peaks in the Afternoon

It’s probably been beaten into your head that the normal body temperature is 98.6 degrees Fahrenheit. But if you take your temperature and it shows a different number, there’s no need to panic. Odds are, it’s a normal change throughout the day, and it doesn’t mean you’re sick.

Research has found that 98.6 is a rather arbitrary number. As long as your temperature is between 96 and 99.9 degrees, you are considered healthy. Your body is coolest at around 6:00 a.m. (an average of 97.6 degrees); and it’s warmest between 4:00 and 6:00 p.m. (an average of 98.5 degrees).