Blame 'Night of the Living Dead' for this, but many people mistakenly think that zombies are nocturnal, going around their business of walking around town with stilted gaits, looking for people whose brains they can eat, only at night.
You think you are safe during the day? You are dangerously wrong!
Remember how one becomes a zombie in the first place? Through death, or Intercision, or, since this is a science blog and we need to explain this scientifically, through the effects of tetrodotoxin. In any case, the process incurs some permanent brain damage.
One of the brain centers that is thus permanently damaged is the circadian clock. But importantly, it is not just not ticking any more, it is in a permanent "day" state. What does that mean practically?
When the clock is in its "day" phase, it is very difficult to fall asleep. Thus insomnia.
When the clock is in its "day" phase, metabolism is high (higher than at night), thus zombies require a lot of energy all the time and quickly burn through all of it. Thus constant hunger for high-calory foods, like brains.
But at night the digestive function is high. As zombies' clock is in the day state, their digestion is not as efficient. They have huge appetite, they eat a lot, but they do not digest it well, and what they digest they immediately burn. Which explains why they tend not to get fat, while living humans with insomnia do.
Finally, they have problems with wounds, you may have noticed. Healing of wounds requires growth hormone. But growth hormone is secreted only during sleep (actually, during slow sleep phases) and is likewise affected by ghrelin.
In short, a lot of the zombies' physiology and behavior can be traced back to their loss of circadian function and having their clock being in a permanent "day" state.
But the real take-home message of this is.... don't let your guard down during the day!
Picture of me as a Zombie (as well as of all my Sciblings - go around the blogs today to see them) drawn by Joseph Hewitt of Ataraxia Theatre whose latest project, GearHead RPG, is a sci-fi rogue-like game with giant robots and a random story generator - check it out.
An utterly shameless plug for my "book store":
Featuring Walking Dead, Walking Dead 2 and Zombie Vegas!
Ah. Zombies are not only ravenously hungry all the time, but due to the insomnia, they are also mega-cranky all the time. If we could get these guys to nap....