Mirrors can be quite helpful as well as harmful if they are not placed in the proper place of your home or business. Below are some do's and don't for mirror placement.
Helpful:
If you have a sloped or tray ceiling over your bed, place mirrors facing up to push the ceiling flat. Best if the mirrors are above your head when sleeping but hidden. Mirrors in your night stands facing up works if you are not able to place the mirror higher. The mirrors will help with headaches or any pressures around your head and neck that you may be experiencing with that type of ceiling.
If you have a toilet/bathroom over your kitchen, place mirror(s) on top or inside your cabinets facing the bathroom to push it away from your kitchen.
Harmful:
The very common mirror that is attached to your chest of drawers/dresser is actually working against you. Mirrors on display are not recommended in bedrooms as they bounce around energy so too yang. Your bedroom should be a quiet, cozy, calm space. At a minimum, cover that mirror over your dresser with a clothe while you sleep.