Just because a sperm cell doesn't have the capacity for reasoning doesn't mean that it's not alive. If that's your criteria for defining life, then life ends where reasoning ceases.

Personally, I think the line drawn between 'life' and 'non-life' is an illusion; everything is alive to some degree (or nothing is, depending on your perspective), it's just that the things which we generally consider to be alive are much more complex.