gwern is an amazing resource and has great articles on a range of topics, including productivity stuff; his posts on nicotine [1], modafinil [2], and melatonin [3] would be 3 highlights.

[1] https://www.gwern.net/Nicotine

[2] https://www.gwern.net/Modafinil

[3] https://www.gwern.net/Melatonin