If you've eaten a good breakfast, and you're faithfully consuming 200-300 calories per hour, and you're faithfully drinking one 750 ml bottle of water and/or sports drink every 1 to 1.5 hours, and you know you're consuming enough electrolytes ........

When you hit about the 5 hour mark of cycling, you're running low on calories. 200-300 calories per hour gives you about half (or slightly less) of the calories you need. After about 5 hours, you're down 1250 calories and you're starting to draw on other resources ... and all of that can make you wobbly and less efficient.

Many long distance cyclists, including myself, like to stop for something more substantial to eat around 5 hours, and repeat every 5 hours or so until the ride is finished. Stopping provides time for a rest and allows you to replenish your energy sources.

If you stop and consume, say, 1000 calories, you will not be able to hop on the bicycle and dash off into the distance, but you can ride slowly into the distance for the next hour or so while the food digests, and then I've found that all of a sudden I have a huge burst of energy for the next couple hours.