Why is it that bread made during the day is lighter than bread made in the early morning
All other factors - ambient temperature, humidity, oven temp, etc - accounted for, the specific time of the day doesn't make any difference, whatsoever.
But alas, this is the real world, we don't hold other factors constant. Most likely your room is slightly warmer and potentially your room could be a bit more humid in the afternoon. These could lead to a more risen product if unaccounted for. But overall, its not the time of the day.
One caveat- There does appear to be some (and by some, I mean a single published paper that I can locate) evidence to suppose circadian cycles in yeast. This would indicate a faster growth rate in the day. But the evidence is underwhelming currently and is almost certainly overpowered by other environmental conditions if present at all.