text: It is generally personal preference.  Morning is good because you do feel more energetic for the day.  About metabolism boost, I'm not so sure if it lasts any longer than it does at night (a healthy breakfast is good for jumpstarting the metabolism).  Some research suggests that greater fat loss occurs when you exercise a long time after eating (be careful not to faint - if you feel faint have some juice or something to get blood sugar back up).  This is because the body is forced to use body stores instead of readily-available blood sugar after a meal.  So the morning may be better for overall weight loss - but I don't think it's too significant and exercising anytime is better than not exercising at all.  Too late at night results in increases in adrenaline (epinephrine and norepinephrine), which can keep people awake.