It is mostly observed in adolescents...
Causes are many...physical, mental related...
It is also a healthy symptom ....
Even it is seen in some health issues...
Next Steps
How to rule out healthy or a problem?
If no fatigue/ tiredness /laziness/ emotional disturbance before and after nightfall ...then it is not a problem...
If it is draining all physical and mental energy then it is problem to be taken care off..
Health Tips
Keep a watch on your mind... physical changes according to age will affect mind too...It is natural....
Always fresh up with a new ideas / knowledge related to your field/ interest.... It diverts the attention of mind.. So that physical change results...