If they hurt/make your teeth sensitive
When beginning, they can make your teeth super sensitive. I recommend starting at night an hour before going to bed, that way any sensitivity has time to go away by the time you wake up the next morning. After the initial period of sensitivity, they seem to hurt less-- so I do it twice before going to bed (for 1h+), and then I don't have to deal with any discomfort while at work.
Rating: 5 stars