Squeak! Squeak! Squeak! How can you expect to wake up rested and refreshed if your bed keeps making funny noises all night long?
Determine the age of your mattress and box springs. The normal lifespan for these items is about 7 years. It will be less than this if (1) the product was purchased at a discount store, (2) you have children who have jumped on it a lot, and/or (3) either of the bed's usual occupants are on the plump side. If the squeak is a relatively new problem, it could just be the mattress and box springs showing their age and needing replacement.
Check to see whether the mattress is centered on top of the box springs. If in the course of the bed being made each day or the mattress being bumped, it may have thrown it off balance and resulted in a persistent squeak.
Remove the mattress and box springs and inspect the metal bed frame. Although these frames are built to outlast multiple beds, they do wear out over time. Check to make sure that all of the screws and bolts are tight. To ensure a tighter fit, take the frame apart and reassemble using plastic washers.
Use a spritz of WD40 as a lubricant to take the squeak out of hinges and metal springs. If you don't have any WD40, you can achieve the same effect with a few drops of vegetable oil.
Check the hardware mountings if you have a headboard, footboard and side rails. This may involve taking everything apart and reassembling it with plastic washers.
Check to make sure that all of the legs of the bed frame are even.
Investigate whether it's the floor that's actually the squeaky culprit. This will involve moving the bed and testing the floor by walking back and forth and trying to locate the source of the sound. If it is the floor, consider rearranging your bedroom furniture so that the bed is no longer resting on top of the squeaky spot.