What's the Best Time of Day to Do Yoga?
In the simplest terms, the best time to do yoga is the time that works best for you. Since the key to accessing all of yoga's many benefits is a consistent practice over time (and hopefully long into the future), you need to find the routine that fits your lifestyle and works with your schedule. This may change over time as your life changes. For instance, you may have gone to yoga classes in the evening right after work for years. But then when you had kids, it made more sense to go during the day while they are at school. It doesn't even matter if you do yoga at different times on different days of the week, as long as you find a routine that is sustainable. Let yoga fit into your schedule instead of trying to work your schedule around yoga.
Some yoga traditions, such as the Ashtanga system of Pattabhi Jois, advocate doing yoga asanas early in the morning, if possible before the sun rises. Many Ashtanga home practitioners stick to this routine and it is most often when Mysore-style classes are offered. (Although I've heard that at the KPJAYI in Mysore there can be so many students that start times for some are pushed back into the later morning, demonstrating that even the master's advice has to be interpreted with flexibility.)
In Light on Yoga, B.K.S. Iyengar advises doing yoga early in the morning or late in the evening, noting that there are advantages to each, saying "Practice in the morning makes one work better at one's vocation. In the evening it removes the fatigue of the day's strain and makes one fresh and calm." Both sound pretty good. Whatever the time of day, Iyengar is very particular that the bowels must be emptied before attempting asana.
While an early morning practice has many things to recommend it, including a compatibility with doing yoga on an empty stomach (and bowels) and the virtue of getting your day off to a good start, it may not be practical for the busy, working folks of the world ("householders," in traditional yoga parlance). Just because you don't want to get up before the crack of dawn doesn't mean you should write off yoga.
You've probably noticed that yoga studios offer classes throughout the day: a 6:00 am class to catch the early risers, a quick lunch-hour class, a 6:00 pm class that caters to the after-work crowd. For kicks, try noticing how poses do feel different at different times of the day. You may be stiffer but have energy in the morning, while you're more limber but also more tired in the evening.
Yoga Time at Home
Finding the right time of day is particularly important if you are trying to establish a home practice. Morning or evening is the most practical for working folks. A morning routine can help you ease into your day and start it on the right foot. An evening practice helps you wind down and mellow out. Whatever it is, you need to know what time is yoga time; otherwise, it's too easy to push it off in favor of things that seem more pressing. Most people are creatures of habit. If you want your schedule to stick, you need to stick to your schedule.
Don't worry too much about anyone else's idea of the "best" time for yoga. No one else is in your body or your life. Find the time that works best for you.