Mental process like waking up without an alarm clock at a specific time aren’t easy. I know a bunch of people who have that skill but it’s not like there a step by step manual that you can easily follow that gives you that ability.
I do not have “wake up at a specific time” ability, but I have trained myself to have “wake up within ~1.5 hours of the specific time” ability. I did this over a summer break in elementary school because I learned about how sleep worked and thought it would be cool. Note that you will need to have basically no sleep debt (you consistently wake up without an alarm) for this to work correctly.
The central point of this method is this: a sleep cycle (the time it takes to go from a light stage of sleep to the deeper stages of sleep and back again) is about 1.5 hours long. If I am not under stress or sleep debt, I can estimate my sleeping time to the nearest sleep cycle. Using the sleep cycle as a unit of measurement lets me partition out sleep without being especially reliant on my (in)ability to perceive time.
The way I did it is this (each step was done until I could do it reliably, which took up to a week each for me [but I was a preteen then, so it may be different for adults]):
Block off approximately 2 hours (depending on how long it takes you to fall asleep), right after lunch so it has the least danger of merging with your consolidated/night sleep, and take a nap. Note how this makes you feel.
Do that again, but instead of blocking off the 2 hours with an alarm clock, try doing it naturally, and awakening when it feels natural, around the 1.5h mark (repeating this because it is very important: you will need to have very little to no accumulated sleep debt for this to work). Note how this makes you feel.
Do that again, but with a ~3.5-hour block. Take two 1.5 hour sleep cycle naps one after another (wake up in between).
During a night’s sleep, try waking up between every sleep cycle. Check this against [your sleep time in hours / 1.5h per sleep cycle] to make sure that you caught all of them.
Block off a ~3.5 hour nap and try taking it as two sleep cycles without waking up in between them. (Not sure about the order with this point and the previous one. Did I do them in the opposite order? I’m reconstructing from memory here. It’s probably possible to make this work in either order.)
You probably know from step 4 how many sleep cycles you have in a night. Now you should be able to do things like consciously split up your sleep biphasically, or waking up a sleep cycle earlier than you usually do.
I then spent the rest of summer break with a biphasic “first/second sleep” rhythm, which disappeared once I was in school and had to wake up at specific times again.
To this day, I sleep especially lightly, must take my naps in 1.5 hour intervals, and will frequently wake up between sleep cycles (I’ve had to keep a clock on my nightstand since then so I can orient myself if I get woken unexpectedly by noises, because a 3:30AM waking is different from a 5AM waking, but they’re at the same point on the cycle so they feel similar). I also almost always wake up 10-45 minutes before any set alarms, which would be more useful if the spread was smaller (45 minutes before I actually need to wake up seems like a waste). It’s a cool skill to have, but it has its downsides.
I do not have “wake up at a specific time” ability, but I have trained myself to have “wake up within ~1.5 hours of the specific time” ability. I did this over a summer break in elementary school because I learned about how sleep worked and thought it would be cool. Note that you will need to have basically no sleep debt (you consistently wake up without an alarm) for this to work correctly.
The central point of this method is this: a sleep cycle (the time it takes to go from a light stage of sleep to the deeper stages of sleep and back again) is about 1.5 hours long. If I am not under stress or sleep debt, I can estimate my sleeping time to the nearest sleep cycle. Using the sleep cycle as a unit of measurement lets me partition out sleep without being especially reliant on my (in)ability to perceive time.
The way I did it is this (each step was done until I could do it reliably, which took up to a week each for me [but I was a preteen then, so it may be different for adults]):
Block off approximately 2 hours (depending on how long it takes you to fall asleep), right after lunch so it has the least danger of merging with your consolidated/night sleep, and take a nap. Note how this makes you feel.
Do that again, but instead of blocking off the 2 hours with an alarm clock, try doing it naturally, and awakening when it feels natural, around the 1.5h mark (repeating this because it is very important: you will need to have very little to no accumulated sleep debt for this to work). Note how this makes you feel.
Do that again, but with a ~3.5-hour block. Take two 1.5 hour sleep cycle naps one after another (wake up in between).
During a night’s sleep, try waking up between every sleep cycle. Check this against [your sleep time in hours / 1.5h per sleep cycle] to make sure that you caught all of them.
Block off a ~3.5 hour nap and try taking it as two sleep cycles without waking up in between them. (Not sure about the order with this point and the previous one. Did I do them in the opposite order? I’m reconstructing from memory here. It’s probably possible to make this work in either order.)
You probably know from step 4 how many sleep cycles you have in a night. Now you should be able to do things like consciously split up your sleep biphasically, or waking up a sleep cycle earlier than you usually do.
I then spent the rest of summer break with a biphasic “first/second sleep” rhythm, which disappeared once I was in school and had to wake up at specific times again.
To this day, I sleep especially lightly, must take my naps in 1.5 hour intervals, and will frequently wake up between sleep cycles (I’ve had to keep a clock on my nightstand since then so I can orient myself if I get woken unexpectedly by noises, because a 3:30AM waking is different from a 5AM waking, but they’re at the same point on the cycle so they feel similar). I also almost always wake up 10-45 minutes before any set alarms, which would be more useful if the spread was smaller (45 minutes before I actually need to wake up seems like a waste). It’s a cool skill to have, but it has its downsides.