I am a 4th year medical student and i have a question about my heart rate..at night my pulse is always below 60, once it was 48bpm..is it normal?

Yes, it can be norma: Heart rates vary over a wide range and are typically slower(10 beats difference) when you are resting as opposed to being up and around. To some degree physical fitness affects the heart rates. Athletes tend to have a lower heart rate, down to 50-60 range and that is perfectly normal. If you have no symptoms during the day and feel well, you can ignore it.

Often normal: Sinus Bradycardia(HR<60 bpm) is a common finding during sleep, where HR can fall to 30 bpm in normal people. It is also common in well conditioned athletes. One medical society considers SB as a HR <50. If asymptomatic no treatment is indicated. It's common for physicians to order an ETT and Holter monitor as a screening study if there is any concern that the bradycardia is of significance.

Sinus bradycardia: you have sinus bradycardia and is of concern if you have symtoms associated with it .like fatigue, dizziness, lightheadedness, faintings etc sinus bradycardia is very common in athletes and persons who jog or run a lot