There's no doubt that vitamin D is essential [1]. The nutrient is known to help build stronger bones, support the immune system [2], and more. Yet D deficiency is very common, affecting about 50 percent of the population worldwide [3]. Some signs and symptoms of vitamin D deficiency [4] include bone pain, muscle aches, and mood changes like depression. But low levels of vitamin D may also cause you to feel downright exhausted [5].

Estelle Darlyse Jean [6], MD, a board-certified cardiologist at MedStar Montgomery Medical Centre, explained that this is because vitamin D is involved in the cellular function of skeletal muscle [7]. Research shows that low vitamin D levels can lead to muscle weakness and fatigue [8]. "This fatigue can be the result of problems in the 'power stations' within each cell of the body called the mitochondria," Dr. Jean told POPSUGAR. Vitamin D supplementation may help resolve this issue, she explained, as prior studies have shown that increasing vitamin D levels in deficient patients can improve muscle efficiency, therefore lessening symptoms of fatigue [9].

Additionally, some research has linked vitamin D deficiency with lower sleep quality [10], sleep disturbances [11], and reduced sleep duration [12], explained Kristin Gillespie [13], MS, RD, CNSC, an advisor for Exercise With Style. It's likely then that healthy levels would help people sleep more soundly [14] for longer periods of time, allowing them to feel more rested.

How Can I Increase My Vitamin D Intake?

Your body naturally produces vitamin D when it's exposed to sunlight [15]. Gillespie recommends getting 10 to 30 minutes of midday sunlight several times a week for that reason — but you can also get vitamin D from foods including fatty fish like salmon and tuna, fish liver oils, cheese, and eggs.

Finally, if you're concerned you're not getting enough vitamin D from the sun and your diet, talk to your doctor about getting a simple blood test to find out if you're deficient. Depending on your levels, they may recommend that you start taking a vitamin D3 supplement.