Diabetes
You could be sleepy from sleep apnea - that is one of the signs - excessive daytime sleepiness. Another cause is too many carbohydrates. If you are fairly sure it is tied into your diabetes, you should test your blood before you take your insulin - and you need to eat at 15 minutes after the shot. If you wait too long your sugar will get too low and that is dangerous. You can also test your blood when you are feeling sleepy to see if it is a dip in your sugar level.