[bcheung]

Not sure about references but the general consensus I have seen and heard, and is congruently in my own life as well, is that willpower is at its strongest in the morning and as the day goes on we experience "decision fatigue" that drains us mentally.

Additionally, there seems to be some kind of slowdown that necessitates sleep. There have been studies (sorry don't have references handy) that show people who are sleep deprived are less likely to exercise self-constraint.

I would recommend the book "Miracle Morning" if you are interested in more.

As to not being productive in the morning, there is a consistency of waking up at the same time that needs to happen. In my experience, it takes 10-20 days before you can function well at the earlier time period.

Also, there are things you can do to increase alertness. Sunlight, drinking water (dehydration makes you tired and mentally sluggish), electrolytes, exercise (just a minute or 2 to get heart rate up), cold shower. All of these wake the body up and increase mental alertness. I frequently use them before I start my day and it makes a big difference. You can't just wake up and start working, you have to have a consistent routine that is aligned with human physiology.

It takes about 30 minutes after getting out of bed before I reach peak alertness.

[strken]

Ego depletion and decision fatigue suffered pretty badly in the replication crisis, although I have no idea what the current consensus is. The key phrase to search for is "ego-depletion RRR", if you're interested.