It is generally considered that morning is the best time to do yoga because it rejuvenates the body and makes it ready for the rest of the day. A couple of minutes of yoga will be a great start to your day while secreting the happy hormones in the body.
Don't worry if you're not a morning person. You still can do yoga at late afternoon or late evening but keeping in mind that there should be at least 3 hours between yoga and last time you are.
Person should be really aware of his/ her body type because there are several asanas in yoga which may suit one or may not suit the other especially for persons having back problems or blood pressure.