Sleep restriction narrows the time you spend in bed to roughly what you're actually sleeping, then widens it again as sleep consolidates. It's a core piece of CBT-I, and in the research it's the component doing much of the work — which doesn't make it easy to sit through. Sleep pressure has to build before it can help, and that means feeling more tired before feeling better.
The articles in this topic cover how the window is calculated from a sleep diary, why the first week or two is the hardest part, and when narrowing the window is the wrong move for a particular kind of bad sleep.