Absolutely exercise when you feel that you will. I'm usually best late in the afternoon but on the weekends I know myself and if I don't do it first thing than most likely it won't happen or won't happen with the intensity it should.
I may not do as well in the morning but at least it's something. Today I ran and I felt very sluggish, much moreso than when I run late in the day, but I took a 5 minute walk break and ran another 20 minutes, overall did about 4 miles...I was shooting for 5 but 4 is better than 0.
Exercise when you want, just exercise