Myself, have submerged, early on in the morning, before the sun was even up... sky had just started to begin to lighten up, indicating that sunrise was not far off... & stayed submerged, until I could see that it was... as totally dark as it could be, by that point... the C02 levels were, I believe (irc) by that time, in the 50-60% range... I could be off a bit... I really wouldn't push it much past 60.
M. M.