I recommend this ^ for picket lining. I have seen it save a horse's life. Plus, for me, it just makes picket lining sooooo much easier!
If you horse is used to being tied for extended periods of time, then he should be fine. If not, what I do is loop a tire innertube over a large tree limb then tie a lead rope to the loops in the innertube. I tie the horse where I can keep an eye on him/her. The innertube gives a little if the horse pulls. This is a good way to teach a horse that just because his/her body can move around doesn't mean he can leave! For some reason, horses that have never been "mobile-tied" as I call it, find it much different than being tied to the side of a stable object like a trailer, arena fence, etc... and need to learn the boundaries of thier movement.
If you horse lays down to sleep, make sure you tie long enough for him/her to get up and down, but never long enough to get a leg over thier rope! I tie my horses long enough to drink out of their water bucket on the ground but short enough that they have to pull the rope just slightly to get that drink, and I tie a hay bag onto the picket line (on another Knot Eliminator of course! Keep the bag from sliding down the picket line into the horses face!)