Not enough hours in the day, is your horse having to pay, the price of its sanity, for convenience.
This article written by Justine Harrison, looks at video, that recorded just 8hrs, of this horse in its stable. What a boring existence! As you can see the horse has nothing to do, only eat if ad lib forage is provided, drink, sleep and poo. I guess its the equivilant of being locked up in the loo, with the flush not working, with no phone, or other internet device, no book and nobody outside the door to talk to.
When you have a duvet day, I bet you have snacks, a box set, a phone or other device for browsing, a good book, or sometimes even good company. It's your choice. Consider what your horse would choose, should he/she choose to have a duvet day.