Parents and spending the night at your girlfriends house

My girlfriend lives about 40 minutes away from me. If there is traffic it can take over an hour to get there. I am often there until late at night and would rather just spend the night at her place, instead of driving back tired in the middle of the night and being at risk of driving while being too tired. The problem is that I still live with my parents, so they are well aware of how late I am gone. So I want to know if I should ask/tell my parents I am just going to stay over at her place. My parents are fairly conservative, and I'm not sure how well they would take me asking. But I come back from her place at 4 in the morning sometimes, so it's not like they don't have some idea of why I am there so late. So should I ask them or just keep things the way they are?
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I feel like they're almost baiting me. I just told my dad I was going to sleep because I'm tired, and he makes some comment about me staying out really late with my girlfriend. I really wish they would just get off my ass about it. I'm an adult and a very responsible one at that.
Parents and spending the night at your girlfriends house
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