Pliny, Letters 9.36:
Around the fourth or fifth hour of the day, (you know, the time was not certain and clearly marked off) as the day urged me, I took myself into the colonnade or the covered portico, where I would think on the rest and dictate my thoughts. I get up into my carriage. There too I do the same thing as when walking or lying down; my application to my work continues, renewed even by this change. I take a little nap, then I walk, then I read aloud a Greek or Latin oration clearly and with attention less for the sake of my voice than for the sake of my stomach; of course, the voice still gains just as much from this exercise.
Ubi hora quarta vel quinta – neque enim certum dimensumque tempus -, ut dies suasit, in xystum me vel cryptoporticum confero, reliqua meditor et dicto. Vehiculum ascendo. Ibi quoque idem quod ambulans aut iacens; durat intentio mutatione ipsa refecta. Paulum redormio, dein ambulo, mox orationem Graecam Latinamve clare et intente non tam vocis causa quam stomachi lego; pariter tamen et illa firmatur.