Athletes often have a very low resting heart rate in the 35-50 range but is this while sleeping? And does it even go lower during sleep?

Yes and ...yes. Everyone's HR slows during sleep, both because metabolic needs are less and also because of augmented vagal tone. Athletes HRs are slow during waking because they have better stroke volume than non-athletes and don't need the faster rate to maintain cardiac output. During sleep, like everyone else, they'll go even slower. Not to worry! Athletes don't slow down and die in their sleep! Honestly!