そろそろ寝ないと。

そろそろ寝ないと。

I'd better get to bed soon.

そろそろ寝(ね)ないと。

Sorosoro nenai to.

そろそろ (そろそろ、そろっと)

slowly; quietly; steadily; gradually; gingerly; soon; momentarily; before long; any time now

寝る (ねる)

to lie down; to go to bed; to lie in bed; to sleep (lying down); to sleep (with someone, i.e. have intercourse); to lie idle

と (と)

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

Readings: シン、ね.る、ね.かす、い.ぬ、みたまや、や.める

Meanings: lie down, sleep, rest, bed, remain unsold