Without understanding your insulin regime it is not possible to answer why you went so low last night. It is not advised so definitely worthwhile working out what happened. Perhaps you can talk to your DSN.
There are all sorts of things which affect our BG but the kind of thing which would lead to a low Bg are alcohol, exercise or too much insulin for the food we ate.
Anything below 4.0 mmol/l is a hypo and must be treated. I was told our brains need 2.8mmol/l to function properly (which explains why a common symptoms of hypos is an inability to focus on one thing). There is a level at which we will go unconscious but this varies per person. If this happens, our livers usually react by releasing glycogen. However, a "hungover without drinking" feeling follows the next day. A bit like your washed out feeling only worse.
If your BG gets extremely low and struggles to recover, you may go into a coma.
Our bodies get used to "normal BG". This is why some people used to high BGs will experience false hypos as they start to get their BG under control. If you experience lots of hypos your body will get used to this and you will not notice when your BG gets dangerously low. This is one of the reasons why hypos are to be avoided as much as possible and treated with fast acting sugar as quickly as possible.
If you are taking insulin you must have fast acting sugar with you at all times (including next to the bed).
Cereal is not fast acting especially if you ate it with milk.
Some people drink sugary drinks (like Lucozade), some people eat sweets like jelly babies (bit not chocolate because the fat slows down the sugar absorption) or dedicated treatments like GlucoTabs.
