I would say... "かなり遅くなったから、寝坊しないように、もう寝た方がいいんじゃない？/ 寝た方がいいと思うよ。" 遅い [ おそい] late 寝坊する [ ねぼう ] to oversleep ～した方がいい [ should... ] Hope this helps. :)

Using the simplest grammar possible: ごめんね、ちょっと疲れた。 Im sorry, I'm a bit tired. - Most people would understand and know that it's time to go to bed. Little more advanced: 遅くなってるから、寝りましょう。 Because it's getting late, let's go to sleep. (although looking at it, it kinda sounds like you want to sleep with that person haahhaa) If I was to translate "it is pretty late, perhaps you should go to sleep so you don't accidently oversleep" かなり遅くなってるし、寝坊かもし、寝ることがほうがいいと思う。 Literally: Because it's getting late, and because we might oversleep, going to sleep is better I think. Or better yet, you could say "the night is finished I think" 今夜は終わりと思う I don't know. That's the beauty of language, you can say something in so many different ways. Anyways good luck with your studies! 一所懸命勉強しなさい！

もう寝るよ、おやすみ。 I wanna sleep. Good night! Don't worry about other people,worry about your health. If you go to bet first, nobody complan abou you. Mosｔ of Japanese will read your mind easily , so you don't need to hide your mind. In another word , no way to hide your mind.

まだねなくて大丈夫？/mada nenakute daijyoubu? This means that you just giving him or her choice of finish conversation.