Staying the night with my boyfriend

I am 18 years old and have been in a serious relationship for almost four years. He has his own place and I would like to start staying the night with him, but i'm scared to ask my (semi strict) mother. She trusts me to go to his house everyday but I have to come home at night and it seems a little pointless since there's nothing we could do at night that we couldn't do during the day.

I'm scared to ask because I have been staying the night with him every now and then but saying that i'm at a friends house. I have a feeling that she suspects I am staying with him but she doesn't say anything. I'm just worried she won't let me stay at "my friends house" anymore and i'll ruin what we've been doing. I also don't want to ruin my relationship with my mom by doing it even if she disagrees.

I just feel like I'm going to have to start making my own decisions at some point.

What do you think?