answer:When you sleep, the brain releases a myriad of inhibitory neurotransmitters. Some are to stop you acting out your imagination, like sleepwalking, others are to stop you remembering what happens in your sleep, since dreams largely contain nonsense information. Some people just have more effective inhibitory mechanisms than others. I never remember my dreams if I get a good nights sleep. However if I am suddenly woken by a phone call or if I am really tired, I may remember tiny pieces of a dream since these things can interrupt the inhibitory processes and thereby allow the information to flow into my memory.