Toni Morrison was a morning writer in the most literal sense. She got up while it was still dark, made coffee, and worked before the sun came up. The habit started out of necessity and turned into something close to a creative rule.

The Direct Answer
Before dawn, usually starting between 4 and 5am.
She rose in the dark, made coffee, and watched the light arrive before she wrote, per her 1993 Paris Review interview.

The Dark, the Coffee, the Light

Morrison described the same small sequence every morning. She'd get up before sunrise, make a cup of coffee while it was still dark, then drink it and wait for the light to change. The darkness mattered to her. She didn't want to be in the daylight when she started. She wanted to be there before it came.

I always get up and make a cup of coffee while it is still dark, it must be dark, and then I drink the coffee and watch the light come.

Toni Morrison, Paris Review, 1993

She called the slow shift from dark to light a signal. It told her she was crossing into the place where the writing happened. As she put it, the point wasn't being in the light. It was being there before it arrived.

Why Mornings Won

When she first started writing, she had small children, and the only reliable quiet came before they woke up around five. So she took it. Later she noticed something useful. She was sharper early. More confident, more clear-headed, more willing to take a risk on the page. By her own admission she wasn't very bright or very inventive after the sun went down, so the schedule stuck long after the kids were grown.

This is the heart of the full Toni Morrison writing routine: a fixed early window, protected and ritualized, that she returned to for decades.

What You Can Steal