When you read "full sun," it means that a plant needs direct, unfiltered sunlight for at least 6 hours a day. This is the minimum amount of light a sun-loving plant needs to thrive. As you have noticed, the intensity of sunlight varies depending on the time of day. A plant that gets full sun all morning but is shaded in the afternoon has a much different growing environment from one shielded from the sun in the morning but exposed to full sun in the afternoon.

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 Many plants that are classified as growing best in "partial shade" can take full morning sun, as long as they are protected from direct afternoon sun. Latitude and elevation play a role too. Gardens in the South receive more intense sun than those in the North. And gardens at high elevations are brighter than landscapes at sea level.