People who suffer from sleep apnea are prone to nodding off during the day — sometimes in dangerous situations, like when driving, as mentioned above. And this is due to breathing troubles that keep them up at night.

"Have you ever heard a sudden, unusually long pause in someone’s breathing during sleep, followed by a gasp for air when breathing resumes?" That may be sleep apnea, Kisch says. "These events can happen dozens, sometimes even hundreds of times per night." And it can lead to extreme daytime fatigue.

"When you’re not breathing, you’re not providing your body with oxygen, so the body wakes itself up to kickstart regular breathing again," Kisch says. "Hardly anyone remembers waking up during these instances, which leads to confusion as to why they’re so tired the next morning."

If you fall asleep at odd times of the day, a doctor who specializes in sleep disorders can check to see if sleep apnea is to blame.

Another health concern that can keep you up at night is restless legs syndrome, which is a disorder of part of the nervous system that causes an intense desire to move the legs, often as a way of relieving a feeling of itchiness or discomfort. If you have it, you likely lie awake for part of the night, leading to tiredness during the day.

"Restless legs syndrome affects about 10 percent of women, is often caused by allergy medications or antidepressants, and is worse in pregnancy," Darlene A. Mayo, MD, FAANS, a neurosurgeon and owner of AVUCA MD, tells Bustle. If you think you have it, let your doctor know.

One of the rarer causes of daytime sleepiness is a neurological disorder called narcolepsy. It's a condition that can be inherited, so you're more likely to have it if your one of your parents does, Dr. Mayo says.

But it can also occur after an injury. "Having a major head injury can cause narcolepsy," Dr. Mayo says. If you're narcoleptic, "episodes can be brought on by having a strong emotional reaction to something," she adds. "This often causes the person to not only fall asleep but to lose control of their muscles."

Most of us don't drink enough water, and are dehydrated as a result. But this habit can also result in fatigue. "Our bodies are made of mostly water (55 percent for women and 60 percent for men)," Dr. Mayo says. "When you don’t drink enough water, you get tired."

If you experience sleepiness, try upping your intake. As Dr. Mayo says, "If you are tired after lunch, ditch the latte and grab a glass of water instead." It may make all the difference.

If you have a food allergy or intolerance and don't know it, it might explain all sorts of weird symptoms in your body, including tiredness. "Hidden food intolerances can lead to a sleepy feeling during the day," Jennifer Woodward, FDN-P, of Reformed Metabolics, tells Bustle. "Most women feel like they need to nap right after lunch — this points to food intolerances."

Eating something that doesn't agree with you can truly impact your day. "For instance, I was having ... organic turkey lunchmeat every day in my salad, only to feel exhausted and irritable an hour after lunch," Woodward says. "I ran a food sensitivity panel on myself and discovered not only am I severely intolerant to turkey, but spinach as well! These healthy foods may not work for everyone’s physiology."

To find out if a certain food is affecting you, speak with your doctor. They can do a blood test, and recommend the best foods for you.

If your diet is unbalanced, for whatever reason, it may eventually lead to a vitamin deficiency, which can cause feelings of fatigue. "Having a low vitamin B12 level is a common cause of feeling tired during the day," Dr. Mayo says. "Vitamin D deficiency is another cause of feeling tired."

To increase these key nutrients, it can help to add certain foods to your diet. "Milk, eggs, poultry, and fish are good sources of both [of these] vitamins," Dr. Mayo says. "Getting some sunshine every day also boosts vitamin D levels. You can have your vitamin levels checked with a simple blood test. Your doctor may order vitamin supplements for you if your levels are very low."

By skipping breakfast or being too busy for lunch, you might end up with low blood sugar — and that can send your energy levels plummeting.

"Having low blood sugar is a common cause of daytime sleepiness," Dr. Mayo says. "This can happen most commonly by skipping meals, especially breakfast, [or] having foods or drinks high in sugar and then not having something else healthy to balance it out. Your body functions best when your blood sugar levels are steady throughout the day. This is what gives you energy."

To keep it all even, "eat small meals or healthy snacks throughout the day," Dr. Mayo says. "Frequent episodes of low blood sugar can be a sign of a more serious medical condition like diabetes." If your low blood sugar doesn't improve, even after eating more frequently, let your doctor know.

Ongoing fatigue may be a sign you need to be checked for anemia. "Low iron leads to a reduction in red blood cells, which can cause exhaustion," Dr. Carrie Burrows tells Bustle.

This is often diet-related, and can affect people who don't eat a lot of iron-rich foods. "A series of blood tests will help you and your doctor figure out what could be causing the sleepiness," Dr. Burrows says.

The folks most at risk for anemia are those with a poor diet, intestinal disorders, chronic diseases, and infections.

Mental health issues often have fatigue as a main symptom, so if you can't seem to shake the sleepiness, it may be a good idea to reach out to a therapist.

"One of the common symptoms of depression is daytime fatigue," Dr. Mayo says. "People who are depressed often have low levels of serotonin, which is a brain chemical. Low serotonin levels can cause fatigue and are also related to chronic fatigue syndrome and fibromyalgia."

Stress can lead to fatigue, too. "When you are under regular stress, your body releases cortisol, a hormone, which makes you tired," Dr. Mayo says. This is why you often need a nap after a super trying day. But ongoing tiredness can also be your body's way of telling you to slow down. If you aren't managing your stress in a healthy way — by doing things like meditating, exercising, relaxing, etc. — it will drag you down.

Hormones can have a big impact on energy levels, or lack thereof. "It is the balance between estrogen and progesterone that is often at fault," Dr. Mayo says. "When there is not enough progesterone in your body it can cause fatigue, because the effects of estrogen become overpowering. This is why women who have PMS, PMDD, or are perimenopausal get tired often."

If you suspect this might be the case for you, let your doctor know. "[They] can check your hormone levels and may suggest diet changes or medications to help balance your hormone levels," Dr. Mayo says.

Often, small lifestyle changes like these can sort your health out, and help you feel more awake during the day. So don't suffer in silence. If you feel tired even after getting plenty of sleep, let your doctor know so they can help you feel better.