Why is my son constantly waking up at approximately the same time every night?

My son is 19 years old, enrolled in his second year of pre law studies at a large university in the north. Through his entire childhood and teen years he had been an extremely deep sleeper; until the middle of last November. At this time he began to experience severe insomnia, which was shocking for its extreme contrast to his previous sleep patterns. It reached its absolute worst near Christmas; at that time he sought the advice of a psychologist,and also began a new relationship, and has been steadily improving;he is now able to fall asleep rapidly, as in the past. However, one very irritating symptom remains: every night between 4-4:30 am, he awakes; sleep usually returns fairly quickly, but nonetheless it is exceptionally inconvenient. When i speak to him on the phone he sounds completely exhausted. Is there something to explain this sudden, and lingering, change? When he returned home for Christmas he appeared to have cystic acne, another sharp transition.. Could the two phenomena be linked? Or is this simply a reaction to stress?