This article is quite old but a more recent measurement from the same experiment (Super-K) used a 1600 day dataset. Of that 1600 days of exposure 860 "days" wer

This article is quite old but a more recent measurement from the same experiment (Super-K) used a 1600 day dataset. Of that 1600 days of exposure 860 "days" were nights. So it's pretty close to half and half. (Cite: Section V-B, bottom left of page 22, of https://arxiv.org/pdf/1606.07538.pdf https://arxiv.org/pdf/1606.07538.pdf ) The daytime data and night time data are decoupled quite easily. Whenever an event is recorded by the detector you just make sure a timestamp is associated with the event. Then you use that timestamp to determine the location of the sun at the time of the event. If the sun is below the horizon it's "night" and if it's above the horizon it's day.