Why do programmers work at night?
Probably both a combination of being more tired and fewer distractions. Most other people will be asleep during the night. Programming is adversely affected by context-switching, thus night-time with stimulant-enhanced soda is more likely to provide you with long periods of interruption free coding.

Update: The above situation creates the conditions where Csíkszentmihályi's Flow is most likely to take place.