'Nocturnal' traces to PIE *nokwts (night) — the ultimate ancestor of both Latin 'nox' and English 'night.'
Done, occurring, or active at night; of or relating to the night.
From Late Latin nocturnālis (of or belonging to the night), from Latin nocturnus (by night, nightly), from nox / noctis (night), from Proto-Indo-European *nókʷts (night). The PIE root *nókʷts is one of the best-attested words in Indo-European comparative linguistics, appearing with remarkable consistency across branches: Greek nyx / nyktos (night), Sanskrit niś, Gothic nahts, Old English niht (night), Welsh nos, Irish nocht, Lithuanian naktis, and Old Slavic nosti. The Latin form nox gives equinox (aequi- + nox, the equal night, when day equals night), nocturne
PIE *nókʷts is one of the most reliably reconstructed words in the proto-language — its reflexes are nearly identical across every branch: Latin 'nox,' Greek 'nyx,' Sanskrit 'nakt,' Gothic 'nahts,' Old English 'niht,' Lithuanian 'naktis,' Russian 'noch'.' The word for night has been spoken, in recognizable form, for at least six thousand years.