Finding time to exercise at any point of the day can be difficult for busy people. You may exercise in the morning or nighttime simply because it fits into your schedule. Depending on your exercise goals, health and lifestyle habits, one time of day may be better than the other for your exercise routine.

Energy
Exercising requires energy regardless of the time of day in which you work out. Your body stores energy called glycogen, which it calls upon for fuel during physical activity. Working out in the morning with little or no food beforehand may be an effective routine for people who are trying to lose weight. According to "Medicine and Science in Sports and Exercise," athletes who exercised with low glycogen levels -- while fasting -- burned more fat than those who trained after having eaten. Exercising at night after eating during the day may be harder to schedule if you are trying to keep your glycogen levels low. Keep in mind, however, that some people feel shaky and lightheaded due to low blood glucose levels if they don't eat before exercising. Most people benefit from a light snack of whole grains and protein an hour before a workout.

Performance
Studies are split in regard to nighttime versus morning exercise and the impact on athletic performance. In one study published in the book "Exercise and Sport Science," people who exercised at night showed higher levels of muscle strength. This is probably due to an increase of testosterone and somatotrophin, post-workout. These two hormones also increase after a morning workout, but not as markedly as in the evening. However, other studies referred to in the book show that the possibility of motor skill improvement in a sport is higher when exercise occurs in the morning because people learn more quickly earlier in the day.

Sleep
Nighttime exercise might not be the best option if you routinely have trouble sleeping. The physical fatigue that occurs with exercise can help you sleep better, but the timing has to be right. Working out right before bedtime stimulates your mind and raises your body temperature, making it more difficult for you to fall asleep. Schedule physical activity for the mornings or at least three hours before bedtime to give yourself time to wind down at the end of the day. If an evening workout contributes to insomnia, try some restorative yoga poses such as the Corpse or Reclining Hero to help you relax.

Air Quality
People who have asthma or other conditions that are impacted by air quality might consider exercising in the evening instead of the morning. Cold air early in the day, combined with high pollen counts, can contribute to breathing problems in people who are susceptible to respiratory problems. Midday exercise is also less safe for asthmatics as well, due to higher levels of air pollution and smog. Better yet, take your exercise indoors and work out at any time of day you like.

DOMS
Delayed-onset muscle soreness is the muscle pain and tenderness some people experience following a workout. In many cases DOMS does not impact an athlete until two or three days after vigorous physical activity. Studies reported in "Exercise and Sport Science" show that people who participate in nighttime exercise experience less muscle soreness than those who work out in the morning. The variance in pain levels has to do with blood levels of an enzyme called creatine kinase, which is a marker for muscle damage. Those who exercise at night had lower levels of the enzyme, but it is not known why.