Q: Playlists only play one song and then stop

Whenever I listen to a playlist from youtube, spotify, earbits, or any online radio stream I can only listen to one song and then it stops playing.  I have a macbook pro.  One song will play and than the screen goes to sleep and that's the end of it.  If i just click my trackpad the playlists will continue for another song and than stop again, it's annoying.  Any suggestions?  Thanks