Why that end works

Because it acts on both systems at once, and it is the only ordinary habit that does.

A fixed rising time holds the body clock in place. The clock is set mainly by light and by regularity, and a wake time that moves by three hours on a Saturday moves the whole system with it. That is why Sunday night is so often the worst night of the week.

And it protects sleep drive. If you get up at the same time regardless, the hours you were awake do their proper work and the pressure to sleep builds where it should. Lie in to repair a bad night and you spend that pressure in the morning, so there is less of it left for the evening.

Notice that this is not sleep restriction. You are not being asked to cut your time in bed to a target. The bedtime end is left alone, and it tends to sort itself out over a couple of weeks.

Every reader asks the same question at this point, so here it is. Is going back to sleep after Fajr blameworthy? There are sound reports of a supplication for blessing on this community in the early part of the day, and a Companion who traded early and prospered is given as the illustration. They support getting up early. They say nothing at all about what you do afterwards.

The positions, and this platform is settling none of them: a number of scholars record staying awake until sunrise as the better practice, and islamqa, in fatwa 2063, holds that no text prohibits morning sleep, applies the default of permissibility, and adds that there is nothing wrong with it for someone who needs it in order to function. Your own scholar can answer your own case. What this course asks of you is clinical rather than religious. A rising time that does not move is what holds the clock, and going back to bed after Fajr moves it, so the advice here is to stay up. That is a sleep argument. It is not a ruling, and if your circumstances mean the morning is the only sleep you get, that is a conversation with a doctor rather than a matter of piety.