Without the sophisticated vocabulary required, babies are poor at expressing their needs and experiences, except through crying. Contrary to old wives' tales, babies do not cry "to exercise their lungs", they cry for a reason. If you have eliminated the usual reasons, hunger, hot / cold, lost dummy, it may be that baby is lonely or even just bored. The answer is contact with a parent - babies recognise their parents by smell as well as sight, so if you think baby is fretting at night, tuck a garment into the cot that has your own personal smell on it - something you've worn that day will be fine. If baby still wakes distressed from sleep, a comforting rock and a cuddle will usually reassure baby that you are not far away, and eventually sleep patterns will settle.