Sleep helps consolidate what you learned during the day.

The idea that 'sleeping on it' helps learning is now backed by decades of controlled experiments. People who sleep after studying remember more than those who stay awake the same interval, and specific sleep stages are linked to consolidating different kinds of memory.

Rasch & Born's (2013) authoritative review in Physiological Reviews lays out the mechanism: during sleep, especially slow-wave sleep, the brain replays and transfers newly formed memories from the hippocampus to longer-term cortical storage, stabilising and integrating them. This is one of the better-established findings in neuroscience. The practical upshot is real: protecting sleep after study is not lazy — it is part of learning.