Most teenagers and college students need between 8 and 9 hours of sleep a night, and that is not a luxury, it is part of how your brain learns. While you sleep, your brain moves what you studied that day from short term memory into long term memory. So a full night after studying is literally when the learning gets locked in. Skipping sleep to study more usually backfires, because a tired brain reads slower, forgets faster, and makes silly mistakes in exams.
The common student trap is cutting sleep during exam season, which is exactly when you need it most. Six hours for a week leaves you as foggy as someone who has not slept at all, even if you do not feel it. Aim to protect your sleep the way you protect study time. It is not wasted time, it is when your effort actually settles in.
If your nights are short, fix the schedule slowly. Move your sleep earlier by fifteen minutes every couple of days rather than all at once, and keep the same wake up time even on weekends so your body clock stays steady.
Example: A student who sleeps 8 hours after revising a chapter will usually recall it better in the exam than one who stayed up till 3 am cramming the same chapter.
One practical tip: Set a fixed wake up time first, even before fixing your bedtime. A steady wake up time pulls your whole sleep cycle into place.