It is caused by overbreathing but can also also occur when you're just thinking about your breathing and manually controlling it. This is because the body usually takes very small breaths, because that's all the oxygen it needs. When you think about your breathing, you have a tendency to think you need larger breaths or breath faster therefore taking in more oxygen than you need. You're still breathing slowly, but because you're mentally controlling your breathing rather than letting your body control it automatically, taking in too much oxygen is the result. Even if you think you are breathing normally just the fact you are controlling it calmly can cause you to hyperventilate. When I say hyperventilate I don't mean gasping for breath I just mean the feeling of not being about to take a deep breath. Because you feel like you haven't gotten a deep breath, you are likely to try to take even deeper breaths. This will just cause you to hyperventilate more, as you continue to take in too much oxygen. Lots of people have hyperventilation syndrome and don't even know - it's only that we have become hyperaware of our breathing. My breathing was totally normal before this started - have never had an issue with breathing/ anxiety/sleeping and then it all hit me at once!