Yes that's what I was worried about. When I came home and looked up heat stress I was pretty sure that's what was going on. Maybe she laid down to nap and got too hot in the sun. As soon as she got cooled down she was acting so much better. I wish I had had a thermometer at the barn to take her temp, I am sure she was higher than normal. Her chest felt hot like when she has been exercised. She was sweating but she normally sweats alot anyway. No skin tenting so she wasn't dehydrated. I think keeping her stalled during the day with the fans (the barn is under some large trees so it stays cooler) then let her out to pasture at night is the best bet until this heat wave relents a bit. It's so hot here that just going outside takes your breath away. Gah... I miss winter, then when it's cold, I miss summer, right now I miss spring when it's nice but not too hot.
Mine will lay in the sun, too, especially if the ground is hot and dry...As long as they are eating and drinking normally I usually let them be. Although it is a little alarming, seeing them out there sprawled in what seems to me an unnatural position.
why would you think that there is a time limit for horses being able to lay down?
Horses very infrequently lay down due to poor circulation - it's why they have locking mechanisms for sleeping standing up. If your horse is laying down for EXTENDED periods of time, then yes, you should definitely be extremely concerned because it is ridiculously easy for a horse to suffer problems that will make it unable to get back up if left for hours on end.