If we could approach our health like a shopping list, most of us would probably wish for a lot of the same things: to avoid illness, to get more sleep, to maintain a healthy weight, and to live a long life while keeping our wits about us. 

Another desire we'd likely share would be linked to all these aspirations: to have more energy. After all, no goal is really achievable if we don't have the energy we need to begin it or to see it through; and few things are more frustrating than spending the day feeling like you're running on empty. 

Here's what most commonly depletes energy levels, what restores them, and the difference between temporarily diminished energy levels and something more serious. 

Why do we lose energy?

The first thing to understand is that any number of things can zap our stores of energy - though it's often a combination of factors that leave us feeling drained.

A big one that affects us is temperature - with heat and sun especially contributing to fatigue. One reason for this is because of thermoregulation - the body's way of maintaining core body temperature by affecting skin blood flow and utilizing energy stores. "This means we have to expend more energy per unit of work to move about in the heat than in cooler temperatures," explains Dr. Anthony Beutler, medical director of sports medicine at Intermountain Health in Salt Lake City. 

A lack of sufficient sleep is another significant factor as getting enough high-quality sleep is one way the body acquires energy to draw from during waking hours. Beutler says that if we don't have enough of that energy stored up, our bodies won't have as much energy to utilize throughout the day.

Poor dietary choices can also lead to diminished energy levels. "Our bodies need to be fed, preferably with real foods at the right quantities," explains Dr. Denise Millstine, a health specialist and director of the Mayo Clinic integrative medicine clinic in Arizona. "Eating a highly-processed, sugar-fueled diet is one sure way to zap your energy." 

Stress and anxiety deplete energy stores as well. Millstine says that too much stress can make you feel tired and that frequently feeling stressed and anxious "can impact how you feel physically and also reduce sleep quality, which further leads to fatigue." 

Certain medications also have side effects that can cause drowsiness - as can alcohol consumption and caffeine crashes related to drinking too much coffee, cola or too many energy drinks. And carrying around a lot of extra body weight can also affect energy levels. "It can feel like carrying around a heavy bag of groceries with you everywhere that you go," says Millstine. 

When should I be worried about lack of energy?

Of course, sometimes a lack of energy can be a symptom of an undiagnosed medical condition. Anemia is one such condition and is related to an iron deficiency that makes it harder for the body to transport oxygen to cells. Heart disease can also affect energy levels. "Some other common conditions related to low energy levels include thyroid issues, depression and anxiety, and autoimmune issues such as lupus and rheumatoid arthritis," says Beutler. 

Diabetes and related difficulties regulating blood sugar levels, plus sleep disorders, cancer and chronic fatigue syndrome are all known to impact energy levels as well. 

Because these and other medical conditions could explain unknown causes of depleted energy levels - especially if you've previously been feeling good - Beutler says it's worth talking to your doctor if you feel anything is amiss. "A sports medicine physician can be especially helpful in looking for metabolic conditions that can cause low energy," he suggests. 

How to have more energy

If you've ruled out contributing medical conditions, your sluggishness can usually be improved by both avoiding the things that deplete energy and doing more of the things that replenish it. 

This includes spending more times outdoors - especially during cooler spring and fall weather - and getting plenty of exercise while you do. "Regular aerobic exercise has been shown to increase energy levels throughout the day," says Beutler. "In most people, it also increases mental alertness for 4-6 hours afterwards, likely due to increased brain blood flow."

Millstine says that breathing exercises can also improve energy levels. "Many find breathwork calming and use it as a way to reduce stress," she says; "and some breathing exercises - such as bellows breathing - can actually wake you up when you need it." Bellows breathing is a yoga-related technique that involves rapidly inhaling and exhaling through the nose in a specific manner.

And because sleep is especially critical for optimal energy levels, Millstine notes, it's important to sleep in a cool, dark, quiet space. "And do your best to arrange your schedule to regularly allow enough time to sleep," she advises. For most people, Beutler says, that means getting at least 8 hours of sleep nightly. 

It's also important to find ways to minimize stress, to spend time with family members and friends who build you up, and to drink enough water. "Nobody feels good when they’re dehydrated," says Millstine. 

Eating a balanced diet may be the fastest way to improve energy levels overall. "Eating regular meals that contain carbs, fat, protein, and fiber is the best way to ensure you have enough energy throughout the day," says Beutler.