You might be surprised how well the brain can filter things while asleep, at least for some people (and will change over time). I have a light alarm clock that

You might be surprised how well the brain can filter things while asleep, at least for some people (and will change over time). I have a light alarm clock that also has bird sounds and one place I lived there were birds just outside my window with the same song as the alarm and they would start a bit earlier than I woke up (hopefully I wasn't disrupting them too much :/). I would sleep through the birds that were at least as loud as the alarm but still wake up to the alarm. I suspect the light part of the alarm helped in that case, but I've had other cases with just very quiet audio alarms that also woke me up, in some cases a radio set to an empty channel at a barely audible level. The light alone does not wake me up most of the time. Relatedly, a friend once told me about a method of just telling yourself when you need to wake up right before you go to bed. I've tried that quite a few times and always wake up within about 15 minutes of the intended time. For me, waking up is the easy part, but I really don't function well at all with little sleep and actually getting out of bed can be almost impossible if I've had little sleep. I can't relate to how anyone can work on very little sleep over a long period of time, although lately I have been doing a bit better on little sleep when not using alarms at all (and predictibly returning to an irregular Non-24 schedule).