If I can't force myself to get up at 4:45am then its a rest day. Happens maybe once every 2 weeks or so. I also commute to almost all my jobs on my bike. I did notice yesterday that after 15 days solid on the bike and 80km between commuting and training that day my remaining energy was ... less than stellar. I can't help but feel guilty if I don't ride 40km or more every day. I'm not interested in winning races or increasing my cruising speed drastically so it seems to work. I have noticed that on days after I've rested I can go faster and further with less perceived effort. I'd say that rest does play a part but I like to go for 10 days or so between rest days.

I'll also take a rest day if I know the next day is going to be hard. Tomorrow I plan on pushing out to 65 miles for the first time this season so this morning I skipped the 30km predawn ride and just settled on the 30km commute by itself.