Sleep Deprivation
It's no secret that parents struggle with lack of sleep when their children are born. Babies don't sleep through the night naturally, and so it takes anywhere from a few months to a year or two for them to learn to sleep all night. If you haven't experienced this, you can imagine what it will do to a person, waking up every night 2-3 times for months on end. Coffee becomes your best friend, the silent third partner in your parenting alliance. It gets better as they get older, but I'm not sure lack of sleep ever stops becoming a part of parenthood. We learned early on that our four year-old is a morning person. About a year ago, we had to set a clock in her room and tell her she couldn't leave the room before 7AM. She was waking up earlier and earlier every day, wanting to play and eat! (A little secret here: we set her clock slow, so it was really about 7:15AM when she was waking up. Don't tell her...) You rest when you can, especially in the early years.