Many studies have tried to pinpoint the best time of day to exercise for peak performance and best results. But most of these studies were designed for elite athletes.
For general fitness, exercise can be whenever it's most
convenient for you. In fact, the best time of day for exercise is
whatever time you can do it consistently. That's because fitness
benefits come from working out on a regular basis.
Consider factors like work and home responsibilities,
your energy level at various times during the day, and what type of
exercise you like best when picking your "prime time" for fitness
workouts.
If you're a morning person whose energy fizzles by 3
p.m., start your day with a workout, even if it means waking up a
half-hour early. If you need a workout buddy to stay motivated, schedule
exercise when it's easiest for both of you. If you like solitude, try
off-peak hours at your gym or create your own at-home workout space.
Remember that you can break up daily activity into three
10-minute segments if that's what it takes to get it all in. Park 10
minutes away from work and walk briskly to the building. Do 10 minutes
of desk exercises at lunch. Then walk back to your car to go home, and
you're done for the day.
Or take a walk during your mid-morning break, your lunch hour and just before or after dinner.
The only caveat is to not exercise too close to bedtime,
or you could become too revved up to fall asleep. Allow at least two
hours before you go to bed -- more if you need longer to wind down.
The American Heart Association has more on when to work out to meet your goals.