The correct answer is that tea is for all hours. Never stop drinking and enjoying tea.
But if you want me to commit to a time period, I'd sooner opt for the evening.
On a morning I usually grab my aeropress and whip up a freshly ground coffee. On an evening though I can reach for any number of low/zero caffeine teas to relax with while not overly caffeinating myself.
You'll often find me sipping a green, white, herbal, or rooibos tea in bed before I sleep. It's just so lovely...