If your bathroom has a ceiling light fixture but no light switch, such as those with pull chains, installing a motion sensor for your light is possible. Light-socket motion detectors are available from speciality stores that allow you to screw the motion detector directly into the light socket, then screw the bulb into the motion detector. The drawback is that you must use a bare bulb, as a shade or globe will obscure the motion detector. Check your local hardware store to see if it sells decorative bulbs that fit your light socket and have the wattage needed to keep your lighting attractive once the motion detector is installed.
Turn off your existing light with the pull chain or other method you normally use to turn it off without a switch. If your light socket has no other method of being turned of, shut off the circuit breaker that feeds power to it.
Twist the socket motion detector into the socket clockwise, as if you were screwing in a light bulb.
Screw a light bulb, either decorative or standard, into the motion detector's socket.
Turn the light back on and adjust the motion settings according to the manufacturer's instructions. Some motion lights allow you to set the length of time the light will stay on.
Cut the chain down to just an inch or two, long using wire cutters. This will keep it out of the way, but you can still use it if necessary by standing on a step stool. You can also reattach a longer chain if you decide your motion sensor light is not what you want.
The light will only stay on for a limited amount of time after motion is detected. You must keep moving within its sensor area to keep the light on.
Some motion-sensing light sockets may not be compatible with CFL light bulbs. Check you packaging and use only approved light bulbs.