Find out where long distance/endurance riders stopped to sleep, from spot trackers?
If you look at the riders who do well in these events, of course they don't sleep much, but beyond that (and not wasting time*) they don't have so much in common though I can think of a few examples where the winner skipped a night early on and kept that lead. Some sleep sooner but shorter. Crucially they only sleep when they really have to, and know enough about what they're capable of to understand how much sleep to get, so:

Planning for a good spot isn't relevant (except basic safety) as you'll want to be too exhausted to care.
Trying to go as far as last year's winner before the first sleep takes no account of different riders or differing conditions (riding conditions e.g. headwind, and sleep conditions  e.g. shelter/not freezing).
Trying to go as long (time) might be better but you won't find that on a map and it still doesn't help with rider differences.
You don't want to nod off on a bike and wake up by crashing hard (though drifting off the road and waking up propped in a hedge seems fairly common).

Of course your sleep kit is inter-related with your sleep strategy - is it almost instant to use? good enough for every night but heavy? only fit for emergencies and catnaps?
Successful riders' own words probably help more than the data. A few examples I've come across that discuss the sleep aspect in some detail

Where there's a will by Emily Chappell (TCR '16 first woman).  Award-winning and recently released in paperback.
I've heard Ian Walker (North Cape 4000 '18 winner) talk about it, probably in this lecture
I heard an interview with Fiona Kolbinger (TCR '19 winner) but can't recall where.


*One I've heard in various versions - if you're not riding, sleeping, or eating you're wasting time.