There are 'forbidden zones' for sleep which are times in Process S 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 onset is governed by the interaction of homeostatic sleep pressure (Process S) and the circadian timing system (Process C). There aren’t fixed “forbidden zones” in Process S where sleep cannot begin. As wakefulness continues, sleep pressure builds, making sleep more likely, while the circadian system modulates when you’re most ready to sleep. The several hours before your usual bedtime is typically a period of rising sleep pressure and often a time when falling asleep is easier, not harder. So the statement is false—the homeostatic drive does not create absolute times when sleep initiation is impossible. Sleep can occur whenever sleep pressure and circadian phase align, though external factors can shift this balance.