At night, we become sleepy and fall asleep. This is because our bodies need to rest slowly in order to recover from the fatigue of the day. Since our bodies are resting while we sleep, naturally our brains should be resting as well. However, not all of the brain rests. A part of the brain is awake and active even while we are asleep. It seems that dreams are created by the brain, which is awake and active while we sleep, playing tricks on us. Sometimes there are days when we do not see dreams, but this does not mean that our brains were not active; it just means that we did not remember the dreams. Most dreams are lost by the time we wake up in the morning, but sometimes we remember them, and that is when we feel that we have had a dream. In terms of time, dreams seen at dawn, just before waking up, when sleep is shallow, are often remembered.
It is not only humans that see dreams. Most mammals, including dogs, cats, cows, and horses, are said to dream.