It's also a sign of being tired!Sleepiness is a sign of low blood sugar.
Also, people naturally feel more tired at about 9am and again at 3pm. I often like a mid afternoon doze! Not on my bike though it has to be said!
Occasionally I feel so. I feel sleepy during afternoon cycling. I just stop it on the countryside and rest under a tree for about half-an-hour and then continue.