What would happen IRL if you ran through the woods in complete darkness? Nothing good, that's for sure. However, in DayZ you may run at full speed with no light at all. At worst, you'll get hopelessly lost. So, why not give a small chance of bad stuff happening to night sprinters? Say, running directly into the tree would give a 10% chance of knockout and/or bleeding. Maybe a bit lower - 5% or even just 1%. Running over logs might break legs. Naturally, running through the fields and cities should be left safe, or people would just hop to daytime servers.
