Sheldon feels comfortable and safe enough to lie down and rest. A prey animal that needs to be able to run to “escape,” is vulnerable when lying down.
Horses need about 2 1/2 hours rest over a 24 hour period, but unlike humans, they can get by if that rest isn’t possible. Due to their size, it isn’t good for them to be lying down for an extended period (hard on their organs) so they often take their rest 15 minutes or so at a time. They have different levels of sleep just like humans.
Horses can rest standing up, usually resting one rear leg at a time. If other horses are present, at least one horse will usually stand guard while others rest.
Enjoy your nap Sheldon!