Peter interviewed Matthew walker. 在我看过”Why we sleep”这本书以后，睡眠这一部分很好懂。他们两个把这个问题讨论得轻松简单：
--sleep stage： REM, non-REM两个stage
--REM sleep 控制激素分泌，包括 -testosterone release
--90min cycle: first part of the light, more non-REM sleep 
--Deep sleep: slow wave, cells all fire and silence together, coma like. Deep sleep can move short-term memory to long term memory. Can lower BP, regulate insulin sensitivity. Lack of deep sleep reduces insulin sensitivity, weaken immune system. 
--If you get up early, you will lose REM sleep. REM: 控制 sex hormone, creativity, executive decision, emotional stability