There are a number of reasons why skin can look better at night. Primarily however, it is probably just due to lighting.
At night there is no harsh light that will show up any imperfections in the skin clearly, even artificial light will not do this as clearly as daytime light will.
Lighting at night is softer and so therefore everything will look softer and cleaner. Different lighting can dramatically change how skin looks.
Women in the 18th and 19th centuries used to only have their portraits painted in the late afternoon to avoid the day's glare.
Another possible reason for this is oils in the skin.
If you are active during the day, then any oil on your skin will be absorbed back into your skin because your circulation is at its best. At night then you would have had a whole day of being active so any oils may have been absorbed back leaving your skin smoother and clearer than it would have been during the day.
Skin reacts to lots of different things, so it could be that yours is reacting to the air and exercise that it has had all day and is giving it a glow.
