Night guards are normally worn when sleeping to reduce unconscious grinding.
Mouth guards are normally worn during athletic sports to protect against injuries of the mouth, or during the day to prevent a grinding or clenching habit.
Custom made night / mouth guards are better than preformed stock guards or the boil and bite types sold over the counter. This is because custom made ones fit better, and in turn athletes are more likely to use them and they stay better in the mouth. Mouth guards are designed individually at different thicknesses for different sports. The greater the chance of impact, the thicker the nightguards made.