Great question, Tom. As tai chi can be both energizing and calming, you might want to practice it earlier in the day to reap the benefits of it in your waking life. However, it does have studied beneficial effects on sleep and insomnia, so regularly practicing it is a good addition to your self-care. A couple of interesting pieces on the benefits of tai chi on sleep:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5034925/ and https://well.blogs.nytimes.com/2015/11/18/tai-chi-vs-psychotherapy-for-better-sleep-and-more/
 Best!