Why will my 5 month old sleep perfectly fine in the bed with me, laying next to me, but won't sleep in his crib?

Misses you: Remember the baby was with you 24/7 and is use to your heart beat, breathing, sounds of your voice, etc. The baby misses you. Also, your body heat is comforting to the baby. Try recording your heart beat on a commerically available machine and put the sounds next to your baby. Be sure to have a baby monitor in the child's room.

Opportunity: A woman has an opportunity at birth to start baby off on the right course.Delivery provides the natural event that makes baby begin to adapt to changes.Putting him into a crib, wrapped up for comfort, allows him to start the process. We loose about 100/yr in the US who suffocate in bed linen. It is never good to sleep with them.You need to start now and let baby learn.

Implicit memory at : 5 mos. leads to habits, learned behaviors, so sleep training is best done at 1-4 mos. If you remain his "sleep association", he'll become a "developmental night-crier". At a regular bedtime, nurse, wipe out his mouth with a soft cloth, read, & put him in his crib in his own room drowsy but awake, to learn to self-calm to sleep. Ignore his protests; they'll get longer & louder at 1st, then stop.