REM periods grow as the night goes on
Across a single night, sleep is organized into repeating cycles, each ending in a REM (rapid eye movement) period. The first REM period is brief, sometimes only around ten minutes, and the REM periods in later cycles are progressively longer, with the final one often lasting much longer.
At the same time, deep slow-wave sleep is concentrated early in the night and shrinks later. So the early cycles are heavy on deep sleep with short REM, and the late cycles are light on deep sleep with long REM.
Why the timing skews dreaming toward morning
Because the longest REM periods fall in the last cycles before waking, the most vivid dreaming and the greatest chance of remembering a dream tend to come in the hours toward morning. This is also why cutting a night short trims away a disproportionate share of REM sleep.
Exact durations vary with age, prior sleep, and other factors, so the values shown are typical rather than fixed. This is general educational information about sleep architecture, not medical advice.