I always hated having to get up from my bed, turn off my lights, then stumble over all the stuff on my floor as I went back to bed. This simple solution takes 5 to 10 minutes, and can be done with stuff from around the house.
Step 1: Prepare your string
Tie a small loop that will fit over the light switch in the string near then end, and tie the heavy thing to the string 3-4 inches from the end.
Step 2: Loop the string over the switch
Loop the string over the switch.
Step 3: Secure it with sticky tack
Secure the string to the switch with sticky tack.
Step 4: Use the thumb tacks as makeshift pulleys
Stick one thumb tack into the wall, and one into the ceiling to use as a pulley
Then attach the end of the string to the wall with another thumb tack
Video of it in action: