Other possible explanations:

1. Low blood sugar - hypoglycemia; or
2. High blood sugar - hyperglycemia.

I have the latter. If I eat or drink something that has a lot of sugar (e.g., candy, fruit, fruit juice, cake, some breakfast cereals) and the sugar rushes into my blood stream, I will have symptoms that mimic falling asleep / passing out, until my blood sugar level returns to normal. To determine if this is the problem, a glucose fasting test is required.