Improve the bracketed part of the sentence.
He is feeling tired as (he was studying since) 5 AM.
Present Perfect Continuous Tense is used to describe an action that started in the past and is still happening or just recently stopped. The sentence clearly wants to say that he started studying at 5 AM and he is still studying that’s why he is tired now so present perfect continuous tense will be used here. Since and for are commonly used in this tense to show the time of action.
For is used to talk about the duration of time e.g. he has been studying for 3 hours.
Since is used for a specific point of time like 1993, 2005, 6’O clock, Monday, etc.
Thus, the correct sentence is: He is feeling tired as he has been studying since 5 AM.