You may feel anxious, your stomach growls, and you might feel shakey. This is the feeling you know all too well. You are hungry and it is time to eat. Risa Groux CN, a functional nutritionalist, explains that hunger is a physiological need to fuel your body with nutrients. Our bodies need food to sustain and thrive, just like a car requires gas to run. She explains that once the tank is empty, the body requires replenishment.

Our bodies make a hormone called “ghrelin” in our stomachs. This hormone signals the brain that food is needed. Ghrelin is a hormone that increases before meals, but drops after eating. However, leptin (another hormone found in fat cells) informs the brain that it has enough energy and doesn’t need to eat any more.

Sometimes, however, this doesn’t always go according to plan. You could even eat more than you need, and still be wondering, “Why am I always hungry?” There are many reasons you may feel hungry all the time, including some common hunger causes and some more serious ones. We spoke with experts to gain a better understanding of hunger and its science.

Some reasons why you are hungry

Groux said that food is fuel and our bodies need it. Our bodies are intelligent and complex. They will tell us when we should eat. Continue reading to learn more about common reasons you might feel hungry.

• #1 You haven’t had a meal in a while

Your stomach is telling you to eat. It hasn’t eaten in a while. Lauren Minchen, MPH., RDN, CDN is a Freshbit nutrition consultant. “You may feel tired or sluggish if you don’t eat. This is your body’s natural response for conserving energy.”

Minchen says that the time it takes to feel hunger pains can vary depending on many factors. People generally feel hungry between three and five hours after eating, but this can vary depending on whether they have eaten a large meal.

• #2 You just went to the gym

After a hard workout or boot camp class, count down the time until you are able to eat a delicious snack or meal. Exercise, especially if it is strenuous or involves cardio, can cause hunger.

Minchen also points out that exercise can cause muscles to be broken down, which in turn leads to a craving for food. She says that training our muscles and breaking it down in order to stimulate the growth of stronger tissue creates a greater need and desire for protein, carbs and fats during this rebuilding process. Our muscles will not become stronger or more resilient if we don’t have these macronutrients.

• #3 You weren’t sleeping well last night

You can feel hungry if you don’t get enough sleep. Kathleen Winston, PhD and RN from the University of Phoenix says that although these may not seem to be connected, sleep is crucial for appetite control. You may be wondering how this works. Our brains and immune system are strengthened while we are in dreamland. This regulates the hunger hormone, ghrelin. Winston says that this hormone can cause increased appetite in those who don’t get enough sleep.

• #4 You don’t eat enough protein, fat, or fiber

Hunger pangs can be caused by not eating enough food. Groux points out the importance of eating a balanced diet with sufficient amounts of fiber, fat, and protein to feel satisfied. She explains that protein regulates ghrelin, and leptin, which helps us feel full. Quality fat aids in the production of leptin, which signals fullness and slows down our digestion.

She says that a high intake of fiber stimulates the production short-chain fat acids. This makes the body feel full and also creates a variety of good bacteria in the microbiome. Soluble fiber is foods that dissolve in water and can reduce appetite, creating a feeling of fullness.

• #5 You’re pregnant or nursing

Minchen points out that the body’s macronutrient and caloric requirements increase during pregnancy to fuel fetal growth. Minchen shares that these macronutrients and calories directly fuel the development of the fetal brain and the growth of skeletal, muscle and fat tissue in order to provide healthy babies with the proper nutrition.

Breastfeeding can also put a lot of pressure on a woman’s stomach and make it feel hungry or thirsty. Minchen estimates that a woman who is pregnant needs an extra 300 calories per day. Breastfeeding can need between 500 and 1,000 calories each.

When You Should Start to Worry

Some medical conditions can cause an insatiable feeling of hunger. To find out why you feel hungry, make an appointment to see your doctor. You may be able to find solutions to your problem by asking your doctor.

• #1 You have a medical condition

Hyperthyroidism refers to a condition where more hormones are in the body than necessary. This can cause excessive hunger, Winston says. Diabetes, parasite infections in the intestinal tract and hypoglycemia are other medical causes of hunger.

Groux points out that fluctuations in blood sugar levels are a common cause of persistent or excessive hunger. She says that when blood sugar levels are high or diabetes is present, hunger tends to increase as well as thirst. This happens because glucose can’t penetrate cells and is eliminated by the body through urine. Hypoglycemia occurs when blood sugar levels are low and blood sugar levels are high. The body will then crave food to regulate blood sugar levels.

• #2 Your hormones may be out of balance

Do you feel the need to eat carb-heavy foods or pizza after a night of heavy drinking? Although this may sound like a normal reaction, it could be a sign that your hormones have gone off track. Groux explained that booze can inhibit the production of leptin, which is the fullness hormone. “Additionally, a higher alcohol intake can reduce the brain’s ability to control self-control. People tend to eat more when they drink alcohol than when they don’t,” she said.

You may also turn to comfort foods or sweets to help you cope with stress. Another hormonal consideration is cortisol, which is a hormone that is produced by the adrenal glands and increases with chronic stress. Groux says that cortisol can increase food cravings and appetite.

• #3 You have an unhealthy eating pattern

Extreme cases of eating disorders like anorexia or bulimia can lead to abnormal hunger patterns. Minchen points out that people with these conditions are more likely to restrict their essential nutrients and calories, leading to a greater sense of hunger.

Their hunger can lead to the destruction of vital tasks and tissue. Minchen warns that skin and hair loss, constipation and rapid heart beat are all signs of a body’s decline due to chronic hunger and restriction.