Cold plunge morning or night depends on what you want out of it, and the three common goals point at different hours. Alertness points at the morning. Recovery points at hours after training rather than immediately. Sleep points away from the evening entirely. There is no single right answer, but there is a wrong one for each goal.
Morning, for most people
Cold immersion produces a sharp, reliable lift in alertness and mood that lasts a while, which is exactly what most people want from a morning. It also makes the habit easy to keep, because a thing you do before the day starts is a thing that does not get displaced by the day. This is where most regular users land.
Around training
If you lift for strength or size, immediately after training is the one slot with evidence against it: several studies find cold immersion then blunts adaptation. Several hours later, or on a rest day, keeps the recovery and mood benefits without that conflict. For endurance work between hard sessions the concern is much weaker.
Evening, usually not
Cold is stimulating, and a spike in alertness an hour before bed is the opposite of useful. If the evening is the only time you have, keep it earlier rather than later and finish with a passive warm-up. A sauna in the evening suits sleep far better than a plunge does, for exactly the opposite reason.
Consistency beats optimisation
The difference between the good slots is small and the difference between doing it and not is large. Pick the hour you will actually keep, make the tub ready at that hour, and stop optimizing. That is also the argument for a chiller over ice: a plunge that is ready when you are is a plunge that happens.
Questions people ask about cold plunge morning or night
Is morning better than evening?
For most goals yes. Cold is stimulating, which suits the start of a day and works against sleep at the end of one.
Should I plunge right after lifting?
Better not. Cold immediately after resistance training has been shown to reduce strength and size gains over a block. Leave hours, or use a rest day.
Does the time of day change the benefit?
It changes which benefit you get. Alertness suits morning, recovery suits a gap after training, and neither suits the hour before bed.