If you read in bed before going to sleep and don't want to keep getting up to turn a light on the ceiling on and off, it is easy to add a pull chain to the fixture. Doing so requires you to remove the fixture, drill a hole to screw on a pull switch and wire the black wires through the switch. You won't need many tools, and it will probably take you less than an hour to complete the project.
Turn off the power to the light switch at the circuit breaker.
Take the light fixture off of the ceiling, remove the wire connectors from the wires and untwist the wires.
Unscrew the bulb from the fixture and pull the bulb socket free. Remove the top of the socket where the wires enter either by unscrewing a screw or by pulling off the top of the socket, depending on the design of your fixture.
Determine where the pull-switch will fit inside the socket and drill a hole large enough for the nut attachment of the switch to fit through. Pass the chain through the hole and tighten the switch to the socket with the nut provided.
Pass both black wires of the switch through the hole. Replace the top of the socket, either pushing or screwing it into place.
Twist one of the black wires from the switch with the black wire from the light fixture and the other with the black wire from the house wiring. Twist the white wires together as they were before. Twist all wires clockwise and cap each connection. Twist the ground wires together as they were before.
Screw the light fixture back into place on the electrical box and restore power to the switch.