When I've had lots of sleep, yet wake up tired, I take another nap about
an hour and a half after I get up in the morning--maybe for 30 minutes to 45 minutes or so. I really try not to, but sometimes the fatigue wins! If I tough it out and am still tired in the afternoon and then rest, I'm afraid I'll fall sound asleep and then be awake at night.
If I feel fatigue in the afternoon, I try to sit for a little while with a good book and just relax. That can sometimes be enough to take the fatigue away, or take the edge off.
I know not everyone here can take a nap, but this is just one suggestion.