Nobody really understands why we sleep. What is pretty clear though, is that something really important must be going on during sleep, because we all do it, we all function poorly if we don't get enough, and the process has been preserved in humans for a really long time. Even when we were sleeping in caves and surrounded by tigers, sleeping was important enough to continue to do. And little kids spend most of their days sleeping.
To understand why sleep problems show up, we need to understand normal sleep. Probably the two most important stages of sleep are stage 4 sleep and REM (rapid eye movement) sleep.
In stage 4, you are completely unresponsive, with a disconnection between sensory information coming in and your ability to receive it. You can't hear people taking to you, you don't know your bladder is full, you don't smell smoke, and you can't tell if tigers are around. This is the prevalent sleep stage of the first part of the night, and the more tired you are, the longer you spend in stage 4 sleep. You have a really hard time being awakened or awakening from stage 4 sleep. Kids with disorders of stage 4 sleep will have sleepwalking, sometimes bedwetting, or night terrors.
In REM sleep, you have a complete loss of muscle tone. That's a good thing, since you dream in REM sleep, and if you acted out your dreams you could really hurt somebody or yourself. REM sleep happens towards the last part of the night. You can wake up easily from REM sleep. Kids with disorders of REM sleep have nightmares.
The stages of sleep, and there are more than just stage 4 and REM, are things we go through each night, but we go through them in cycles. Every 90 minutes of so we actually wake up, check out our surroundings and go back to sleep if everything feels safe.
So, as sleep has developed, one of the major issues surrounding it is safety. We could comfortably enter stage 4 sleep because other people were around us to keep us safe, and at least one person was probably in lighter stages of sleeping so that they could warn us if tigers were around. That sense of connection is really important to us now even though we aren't really worried about predators. Disconnection from your sense of safety interferes with sleep.
Awakening about 5-8 times a night is normal. We don't remember it, but we adjust our pillow and covers.
Infants sleep about 20 hours a day. They frequently are awake at night, much as they were when they were in the womb. Just because we changed their scenery doesn't mean they know day from night. They can snuggle with us and sleep in our arms and we aren't setting them up for anything bad. We can't spoil kids in this age group and I highly recommend holding and snuggling as much as possible :) They should sleep on their backs not on their sides or belly. The hormones that regulate the day night cycle kick in about at 6-8 weeks of age.
When they are about 3-4 months old, they have decrease in night waking. That decrease is a brain development phenomenon, and has nothing to do with solid foods, rice cereal or quality of breastmilk.
At 5-9 months of age, there is an increase night waking again. Many kids need their parents to feel safe because the brain pathways that sense strangers and fear are kicking in. The whole idea of kids sleeping in their own room, in their own bed, is a 20th- century, pretty -much- American idea. You see, most species, since they all sleep, sleep in groups, with each member of the group in different stages of sleep so that predators don't sneak up on them without somebody knowing about it. And in many parts of the world, families sleep together and the kids are never by themselves. More on cosleeping here.
These happen in the first third of the night, where stage 4 sleep is the predominant stage of sleep. Remember this stage of sleep is characterized by lack of sensory input to your brain. Kids who are sleepwalking or having night terrors are really hard to wake up and they will not have any memory of what is going on. Night terrors are screaming scary things where the child acts as if they are awake, but you as parents can't wake them up and in the morning the kids have no idea that anything happened. They are different from nightmares, where the kids wake up easier and remember the dream. And in contrast to kids having nightmares, kids having night terrors are very resistant to having mom and dad trying to comfort them. In fact, if they do wake up, they are really confused as to why mom and dad are in their room in the first place, and since we look scared, they are too.
What happens is that at the end of the first "cycle" of sleep, part of the brain wants to wake up and check safety. The other part of brain says no, I'm too tired. Part wakes up, other part doesn't. And depending on what part of the brain wakes up, the kids can look as if they are awake. The peak age for these things to occur is when kids give up their nap.
Overtired children have more problems with partial arousals since when they are overtired they need more slow wave sleep. Falling asleep tense and anxious encourages this brain "split:" there is part of the brain that really wants more sleep and part that knew that the world wasn't really safe when it went to sleep and wants to wake up. We treat it by increase sleep during the day and having positive emotions around them as they fall asleep. If sleepwalking or night terrors do occur, just keep the kids safe. You do not have to wake them up.
This is bedwetting happening in the first part of the night. It works similarly to the night terror explanation. The kids sleep soundly in stage 4, they get to a part in the sleep cycle where they are supposed to wake up and check for tigers but part of the brain really wants to sleep. The bladder thinks (if bladders think) that it's waking up, the brain says sleep, and we have bedwetting. The key here is to get the kids up , in the first part of the night, and take them AWAKE to the bathroom. Too many people drag a very soundly sleeping child to the bathroom, which doesn't help the kids much. In order for this to work, the kids have to be aware of what they are doing. This type of bedwetting is usually hereditary and eventually, the kids will wake up enough to sense bladder fullness.
Snoring can really mess up sleep patterns. The kids who snore have to keep part of their brain awake to support the muscles of the upper airway during sleep, so that they don't obstruct their airway and stop breathing. If you have to keep part of brain awake, you never get a good night's sleep. Kid sleep deprivation is more likely to be hyperactivity and poor concentration than other stuff we as adults experience. In fact, snoring has been shown to affect school performance, and "heroic" snoring can cause enough kid sleep deprivation symptoms during school that it has been mistaken for ADHD.
Kids who snore usually have big tonsils. Big tonsils that cause kids to snore like men may need to come out.