Nursing can be very demanding in all areas - physical, mental and emotional. You'll typically be on your feet all day, and heavy lifting is often required. There are days that are so busy that there is no time to eat or to even go to the bathroom. Decisions you make can have huge consequences, both positive and negative. You'll be forever assessing and re-assessing, constantly using your critical thinking skills. You'll have to admit that you can't save everyone, and will have to watch people die while providing understanding and support to their loved ones.
It can be the best and most rewarding job, and can be the most thankless and challenging job at the same time. Most nurses will tell you that when you can make a difference in someone's life, it often makes the difficulties worth it.