Why not take a short nap? I'm putting in practically the same miles and I too have succumb to this afternoon slump. It doesn't happen every day but when it hits, I just go ahead and nap (usually 30 minutes to an hour). When I wake up, I usually find that I'm ready to get back out on the bike. If you don't have time for a nap, then... well I don't know. Just take a nap.