A rota, caring responsibility or repeated interruption can leave little control over when sleep happens. A standard bedtime routine may be impossible.

Sleep pressure and circadian timing still matter, but the practical first question is what sleep opportunity and support are actually available? A technique cannot compensate indefinitely for too little opportunity. Record constraints without blaming yourself for them.

For shift work, fatigue also needs discussion with the employer or occupational health team. It is not solely a personal habit problem.