Good Afternoon and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
It appears, Sally, that your horse may have a rare condition called narcolepsy. This is a rare condition that causes them to fall asleep for no reason.There is no cure and the epsiodes may happen frequently or not often. There is a defect in the brain that causes this. It is best, Sally, to have your vet examine your horse to determine if this is a possibility. Some cases can be very dangerous for the rider.
This LINK may explain better.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, Dr. Loretta