There are 'forbidden zones' for sleep which are times in the circadian phase when it is difficult to initiate sleep, such as in the 2-3 hours before the habitual sleep onset. This statement is:

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Sleep timing is governed by the body's circadian clock, which creates windows when it is easier or harder to fall asleep. There are periods in the circadian cycle where the brain is more resistant to sleep onset, often called a forbidden zone for sleep. During these times, initiating sleep is biologically more difficult even if you feel tired, because wake-promoting signals from the circadian system and brain temperature patterns are still outweighing sleep pressure.

The example given—needing to sleep during the 2-3 hours before your habitual bedtime—illustrates this idea. That window can fall within a transitional phase where the circadian drive to stay awake hasn’t fully shifted toward sleep yet, making sleep onset more challenging for many people. Recognizing these zones helps explain why timing sleep attempts to align with the circadian sleep window, rather than trying to sleep during a period when the body is not yet ready for sleep.