There's something it it though that's not interchangeable, that becomes a bigger factor as you age (see: that Bezos quote). The brain is becoming more physical

There's something it it though that's not interchangeable, that becomes a bigger factor as you age (see: that Bezos quote). The brain is becoming more physically tired during the day. At a certain point, say 5pm, it's done: the accumulated wear and tear is too much. Try again after sleeping. This might not be a concern at 20 but is at 60 (Bezos' age).