My horses come in at night and everyone will lay down at least once during the night.
They all MIGHT lay down once during the day.
That being said, two years ago my then 19 yo TWH started laying down a lot in the early spring.
In my infinite wisdom I thought he was laying down to take in the warm spring sun and that he was getting older and just wanted to lay down. All my current horses have historically had extra lay down time in the spring warmth.
That was NOT what was happening with my 19 yo. He was sick with Equine Metabolic Syndrome. It took him losing around 80 pounds in 6-8 weeks time for me to smarten up and get the vet out. Fortunately he never foundered.
He is now 21 and is back to lying down his normal amount of time but, like you, I am a nervous wreck and I time how long he lies down if I am home.
I think I should probably work every day instead of part time so I don't monitor his rest time. It would save me from needing a salt lick full of Zanex