There are a few reasons why shoulder pain is worse at night. Firstly, during the day you are moving around and doing your day to day tasks. This keeps the shoulder a little more mobile and pain is not as noticeable. However at night, you are not moving around as much and this causes the shoulder to stiffen up more. Secondly, at the end of the day, it is likely that there are less distractions than during the day which makes you more aware of the pain.