Whey protein vs casein
Whey protein is a fast protein and casein protein is a slow one. That difference is real and it is smaller than the internet claims. Total daily protein, in the range of about 1.4 to 2.0 grams per kilogram for people who train, is the decision that shows up in the trials. A casein shake at night is optional. It is not required for the gains, and it is not a fat-loss drug.
Nighttime casein is a schedule, not a different goal. Whey protein empties from the stomach faster. Casein protein clots and digests more slowly. Both are complete proteins.
What the protein evidence actually ranks
The ISSN stand (PMID 28642676) spends its attention on daily intake, distribution across meals, and training. It does not require casein at bedtime. Morton's meta-analysis (PMID 28698222) ties extra daily protein to better gains from lifting, then shows the benefit flattening as intake rises. A slow protein at 10 p.m. cannot fix a day that was short on protein, and it adds little if the day was already sufficient.
When the slower protein is the point
People who wake hungry, or who struggle to eat enough at dinner, sometimes prefer casein because it is more filling. That is a preference. Lactose intolerance, milk allergy, and a need for a plant protein are reasons to leave both dairy proteins. A head-to-head product page on this site is whey vs casein.