why at the beginning of spring i feel very tired?!

Exercising a little bit everyday may help you feel more energized, also eating a healthy ish diet with plenty of fruit and veggies in there will give you more energy and also gettting enough rest on night times, even if you don't actually sleep. On the other hand though, concentrating too much on your studies and doing too much work at once without having regular breaks could also make you feel like this.

What i find is if i have been busy throughout the day and then sit down and do nothing, i get really tired and start feeling very sluggish, do you find the same? Maybe going for a walk or cycle to get some fresh air will wake you up a bit too, i've tried this and it does work.

In fact, this is the human nature of climate change with the seasons of a response.
Experts point out that this is not a disease, nor is the lack of sleep, but the physiological function of natural climate change occurs as a physiological phenomenon, is the result of seasonal changes

I am always like this too when they change the clocks. If you are tired then get a bit more sleep - it will soon pass.