Well, I would not call the security situation in Sri Lanka entirely stable - so you never know what will happen in the future.

While it is safe to assume that SQ operates during the day for safety reasons, you can't tell if they do not alter their schedules at a later time.

So unless you have to be in CMB for another important meeting as well, you might want to reconsider travelling through CMB if you really can't afford any kind of schedule change. Or maybe you can live with that small remaining factor of uncertainty, and factor in some plan B how to alter your travel plans so that you can attend to your meetings if anything happens (and that could also be some surprise element like bad weather or some technical problems with the plane).