Sleep
I should add Why We Sleep to my list of influences.
At a friend's gathering on Monday, I left early just before dinner was served. I went to a hotel and slept for 10-11 hours. Because I only had 4-6 hours for the previous few nights, this was a magical decision. I felt great the next morning.
Earlier this year, I would not have made the same tradeoff against socializing. But now that I have a mission, being well rested and focusing on my work is a joy. Strangers usually lack context and are not particularly additive.
Bryan Johnson personally came to tell us at the All-in Summit how sleep is his number one recommendation.
EDIT - also worth checking out "Matthew Walker's "Why We Sleep" Is Riddled with Scientific and Factual Errors" (11/30/25)