Letters 15.5

But whatever you do, come back soon from body to mind. The mind must be exercised both day and night, for it is nourished by moderate labour; and this form of exercise need not be hampered by cold or hot weather, or even by old age. Cultivate that good which improves with the years.

Seneca, Letters to Lucilius 15.5, translated by Richard Mott Gummere