Have you made any changes in your diet or lifestyle recently? Are you eating adequately especially complex carbohydrates to keep up the energy levels. If you go to the gym, make sure you eat pre and post workout meals with sufficient carbohydrates and proteins as exercise increases your need for energy and proteins. Also keep yourself well-hydrated throughout the day by drinking enough fluids to prevent lethargy. A lack of B complex vitamins in the diet also causes low energy levels. Make sure you include whole grain cereals like whole wheat products, bran containing foods, whole pulses and legumes. Have a good breakfast in the morning with plenty of fibre from fruits and whole grains. Sleep well at night and avoid sleeping during the day. You can take a nap in the afternoon but sleeping for long hours or staying in bed for long hours causes laziness and lethargy due to low food intake and disturbed meal pattern. Avoid skipping meals or too much gap between two meals which cause low blood glucose leading to low energy levels. Hope this will help you to boost your energy levels.