NightShift is probably done using ColorSync. When slowly transitioning from day to night, the whole color transformation tables need to be updated over and over again and the cocoa change notification need to be transported to all active applications during the time of the change which in turn would again change their appearance over and over again. Even if this notification will not be parsed, all application need nontheless be woken up for a short period of time. To make the change not noticeable, there would be several iterations, draining unnecessary power. As we all know, the low power mode is here for precisely hindering that. Therefore disabled. They could theoretically not fire a notification during low power mode, but therefore creating the need for cached notifications getting active as soon as an application gets active. But this - to my knowledge - does not exist in the cocoa framework. Maybe in a later version but it looks like they are not there yet. That's my guess.