I wouldn't consider one bad day to indicate a problem overall. Even at my peak of health and fitness when I was in my teens and 20s in the 1970s-'80s I occasionally had days with no energy, or experienced blood sugar bonks despite eating well.

While I enjoy variety in my diet, to minimize variables I eat the same thing before every ride, day or night: oatmeal with yogurt and banana slices, and a couple of cups of coffee. That's usually good for a 60-90 minute ride. If I bonk or don't feel well, it's not because of what I ate. There's something else wrong.

Often it's related to stress, lack of adequate rest. Sometimes a low level health issue that isn't readily apparent, like a simmering infection that hasn't become obvious enough to demand treatment. Last year I found myself dragging more and more over the summer. Turned out I had a sinus and tooth infection that didn't clear up until the cracked molar was pulled and I got antibiotics.

And as the primary caregiver for my elderly mom who's suffering from dementia and other ailments, I often don't get enough sleep. I'm usually lucky to get 2-3 hours of uninterrupted sleep a night and have to make up for it with short naps. Parents of babies or any demanding kids, especially single parents, will have the same experience and challenges (although it's a bit easier to cope with when you're younger).

So I take it easy on bike rides when I'm not well rested, even if I've eaten properly. I take plenty of time to warm up gradually, usually riding easily for 30 minutes before tackling any serious hills or trying to do sprints or interval training.