I can't wake up when my baby cries.

I'm a first time dad and normally I'm not a particularly deep sleeper, but now I'm so exhausted that it takes my SO kicking me to wake me up when our 2 month old cries. She blames the fact that I used to go to sleep late most nights, but that's definitely not the case since the baby was born. I also don't see how that affects the current situation since I've had 2 months of a messed up sleep cycle. She also thinks I should wake up out of instinct just like she does or that I have to train myself, but I don't know how and she refuses to help.What makes it worse is that when I do wake up I'm really groggy and it sometimes takes some time before I'm of any use. I'm not able to sleep during the day at work and I come straight home to help take care of baby. I think I get about 4 hours of sleep most nights. The baby actually sleeps on the bed with us.One night my SO Woke me up and asked me if I heard the baby crying, I said yes. She asked me why didn't I get up, and I said because I was sleepy. I have no recollection of this exchange, I hope that if I actually heard the baby crying that I would get up.My SO is very worried that when she starts work in December, that I'll just sleep through baby's cries. What can I do to wake up? She doesn't think she can take being the only one that wakes up during the night for much longer. When she does get up, I help with burping and I'll take care of baby while I'm up so that my SO can get some sleep.Sorry if I seem like I'm rambling, but we're both frustrated. via /r/Parenting http://ift.tt/2cM0Hxm