If you get in the habit of riding, you don't really think about other options. Getting on the bike is just part of the morning routine. I'm not really who you're directing the question at, but it's not hard for me to imagine.

My guess is though that a lot of people who DO depend on their bikes have other options they can fall back on. I would think most people do regardless of how they get to work. When I commuted by car every day I had alternatives if my car was getting worked on.

Your questions seem to revolve around commuting while in a physically degraded condition, either because of exhaustion or illness. It's not that unusual for me to be up late working on a project, ride home at 2:00 or 3:00 am and then turn around and go back again at 7:45. Which leaves me 3 to 4 hours to sleep. Here's the secret: Getting out of bed sucks either way, but I'll be more alert if I ride to work than if I drive. And in my case, for most of the year I don't really save any time by driving. Winter is the exception.

As far as illness goes I'm just lucky I guess. I get colds and stuff, but it's rare that I have anything bad enough that will cause me to miss work. If I'm well enough to work, I'm well enough to ride. If I'm too sick to ride, I'm not going to get anything of value done at work and I'd be better off staying home in order to recover anyway. A cough isn't enough to keep me off the bike.

Usually for me commuting is no problem at least 4 days a week. I do drive on occasion more because of logistics than anything else. I'll have to pick up the kids, or need to be some place after work that I can't get to quickly enough on a bike.

Are you asking this question just out of curiosity or are you having trouble making commuting a habit yourself?