Yes, you can refuse to do night shifts, but you will likely need a good reason for doing so, because employers may generally not take very well to an employee wanting to leave the night shift for no good reason. When you get hired, you may not have gotten hired for any one shift or necessarily on a particular unit or floor which means that the employers are free to change conditions of employment as they see fit, however, most governments have provisions for the employees to be working in a situation that is not hazardous to their health, and if you can show that it is hurting you and your productivity, the employer may consider changing your shift.