Do you keep your CO2 going during the siesta?
I have an open top tank and run the co2 non stop. I've been doing that for over two decades. Light also varies because most of the lighting is natural with some supplemented lighting. During the Spring-early Fall it's all natural lighting. Not once have I had a problem with co2 gassing of fauna. The key here, may be do to co2 having an easy way to escape in contrast to a closed topped tank.