I have been advising my patients to give the baby a bath between 8-9 PM, in order to signal the end of the day and as a way to relax for the night. The day-night rhythm (circadian rhythm) is not established when the baby is first born, and it takes about 6 weeks to start to develop. Just about the same amount of time it would take an adult who starts working the grave yard shift to get used to the day/night change.
By about 6 weeks of age, the baby starts showing signs of fussiness and cluster feeding during the evening hours, when the sun is going down. That is the first indication he/she is starting to adjust. The evening bath will eventually relax the baby after the fussy period, preparing him to sleep longer at night. You don’t need to wash the baby with soap every night, if you’re worried about drying the skin. Just nice, warm water over the whole body, including the head, is enough.