How do I measure my sleep?

If I am going to really fine tune my sleep (think A/B testing) then how will I measure my sleep?

Eliana suggested I get an Apple Watch. Seems like the first solution I will take since I have high confidence in Apple products - it’s going to be an expensive option but I for me I can afford it and Apple is like Tesla and Dyson - they make expensive premium products but if you can afford them then tend not to disappoint in my experience.