I have heard a lot about benefits of HIIT; however, nobody seems to address when is the best time for such activities. I got this question when watching "The secret life of your body clock" (BBC Horizon 2009). I intend to do HIIT in the early morning to kick start my metabolism and also to have a boost of endorphins for my day. However, the BBC documentary suggests that the best time for exercise in general is afternoon and it might not be safe to exercise heavily in the morning!

Do you know of any scientific study addressing this issue? If not, what is your personal opinion (i.e. would you do HIIT in the morning)?