I started exercising after work, but sometimes I would wonder if I had anything left to exercise with. My solution for tiredness and blahs is to eat a snack before I leave work for the exercise session. I try to have it between 3.30 and 4, but sometimes it is later.
I find protein is the most sustaining for this. My snack might be some cheese, cottage cheese with a little fruit or a boiled egg, or even a cup of soup.
The up side of exercising later is that it curbs my evening appetite. If I get home at 8.30 all I really want to eat is soup. If it's been a hard workout, sometimes I don't want to eat at all, I might rather go to bed.
Then I wake up very ready for breakfast!