Reducing insanity..

Hey guys, I am a 5'9 140 pound college student and I have been doing the month 2 circuit of insanity for 6 days a week for quite some time, and the last month or two I have included extra workouts at the end and have been working out for around an hour and 45 minutes. I am considering reducing the amount I do this because I do it early in the morning and by about noon or so I absolutely have no energy for the rest of the day. I'm just afraid that if I cut back to 5 days and shorten my workouts a little bit that my metabolism is going to slow way down and I'll start putting on fat. Is this a legitimate fear or is backing down the workout some a good idea?