It is not optional.

The ability will only trigger if you have 5 or less life at the beginning of upkeep. This is checked again at the start of resolution and nothing happens if you do not still have 5 or less life.

On resolution of the trigger you have to sacrifice this card if you have 5 life or less. If you can't (because it is not on the battlefield, for example), then you still take an extra turn after this one.

If multiple "extra turn" effects resolve in the same turn, take them in the reverse of the order that the effects resolved. In other words, the most recently created extra turn is taken first.