If you wake up after 4-5 hours of sleep and cannot fall back asleep, it is likely due to an offset in your sleep pressure (adenosine); it is crucial to fall back asleep because the final hours of sleep are essential for the consolidation of learning.

noli's read

The link between adenosine and sleep pressure is a well-established physiological mechanism in chronobiology research (academic journals on sleep homeostasis). It is true that sleep structure changes throughout the night: late-night cycles are richer in rapid eye movement (REM) sleep, a phase intimately linked to the consolidation of procedural and emotional memory, as confirmed by various meta-analyses on cerebral plasticity. However, asserting that the inability to fall back asleep is *solely* due to an adenosine offset simplifies a complex reality, as cortisol, blood glucose, or stress can also play a major role. The idea that these specific hours are the only ones where learning-related brain changes occur is a slightly exaggerated interpretation, as consolidation happens throughout sleep, although functions differ according to the stages. In sum, the advice relies on solid neurobiological mechanisms but omits the multifactoriality of nocturnal awakenings.

the sources

No named source held this one up — that absence is part of the verdict.