The sleep cycle (see Sleep Facts) shows us as having periods of sleep that are lighter and some that are heavier. During the lighter phases of sleep, partial or full awakening can happen and this can be caused by a number of factors, some physical, some sensory and some emotional.
Physical factors might include needing the toilet or a change to the temperature of the room. Physical discomfort may be related to specific conditions and could include feeds, needing to be turned, muscle spasm, incontinence or breathing difficulties.
Sensory changes might be things like a night light being turned off, blankets falling off or waking up somewhere different to where you fell asleep.
Emotionally, your child might be experiencing stress or anxiety about any number of things that have happened in the day or even anxiety about sleep itself and being in their bedroom, without the person/people/things that make them feel safe and secure.
Night waking is common to all of us but settling back to sleep is often the issue. Take a look at what these resources have to say about night waking.