On your home page, you'll notice the health bar. You'll probably notice this stays at a strict 60% balance. That's because you've not set your priorities to be higher!

Every night at 10 minutes before midnight, you receive a base 400 Bux for your breeder services. Of this, some money is spent on keeping you alive. The money spent is called "priorities". You're tallied up for how much each priority costs to maintain every night. So you make 600, and spend some on fancy food and never bored entertainment. 600 - 150 (normal basic needs) - 300 (fancy food) - 300 (never bored) = -150. So you lost 150 bux that night, however you also gained some health.

You can live perfectly fine at 60% health, with just 150-bux priorities. However, 60% health will make it hard to sacrifice well for single-splice monsters. It's generally considered if you're going to move up a tier, that you do it all at once and not just add luxurious basic needs to trashy food. In fact, if there are two tiers apart from each other, the lower tier will negate the higher. The in-game reasoning for this is that just because you spend 750 bux staying warm and having nice clothes doesn't mean that eating scraps for food isn't going to make you magically you shoot up to 70%. So once again, if shooting for new tiers of health, it is advised you set all priorities to the next tier also.

Tiers

300 Bux/night - max 70% for both health
500 Bux/night - max 80% for both health
750 Bux/night - max 100% for both health

Note: 90% is also a tier and can technically be achieved by choosing only two top priorities.

Breeder Experience

Finally, you get some breeder experience every night. Your health = how much you get.