Mid-day crash. There are many possibilities, such as not enough sleep the night before, too much caffeine or sugar in the morning, and medical problems like thyroid disease or mono (to name a couple medical problems). Another possibility is if you are on prescription medications, the crash might be when the meds are peaking or troughing in your bloodstream, depending on the med. Ask your doctor to check on it.
Narcolepsy. Do you actually fall asleep and have no control over that? If so, you might want to talk to your doctor about suffering from narcolepsy.