How often do you take a really deep breath during the day? Perhaps once or twice when you get winded from overexertion?
Getting into the practice of deep breathing can increase your energy level by boosting the oxygenation of your blood and cells. It can also help with elimination of toxins through respiration, as well as reducing your stress levels.
Inhale through your nose as deeply as you can. Hold your breath. Then exhale through your mouth, pushing as much air out of your lungs as possible. Try to achieve a ratio of 1:4:2. In other words, if you breathe in for a count of 4, hold your breath for a count of 16. Then exhale for a count of 8. Work up to doing this 10 times in a row, and then shoot for doing 3 sets during the day. Slow down or shorten your counts if you experience any dizziness or if you feel lightheaded.
You can overcome many simple afflictions like sluggishness, lack of focus, and lethargy simply by increasing your lung capacity and getting more oxygen into your body.