Background: Agent Jones is pretty close to being peak human: in physical contests he wouldn't win against an Olympic medalist in their discipline but he's easily above the 95 percentile when it comes to fitness. In terms of mental skills, he's sharp and has two decades of exemplary in-the-field secret agent experience. He also lives a very regimented life with exact rituals concerning what, when, and how much he eats, how long he sleeps, when he exercises, when he shaves, etc. Needless to say, he knows himself and his body very well.

The Scenario: Somehow, the evil bad-guy syndicate gets the jump on Jones, and the last thing he remembers before waking up is getting manhandled into a private jet before fading into a drug-induced unconsciousness. Now he's woken up in an interrogation room.

The Question: Being as in-tune with his body as Agent Jones is, can he estimate how many hours or days he was knocked out by the amount that his hair, specifically the stubble of his normally clean-shaven chin, has grown? What level of accuracy can be expected for this estimate?


2 Answers 2


Running my hand along my face, a few hours after shaving, I feel I can detect the individual hairs. Waiting 10 hours, yeah I can feel more. Next morning, oh lots more. I'd be very confident that I could learn that pattern, especially as it grabs on my hand as a drag it over it, and use it to estimate with good certainty how much time had past.

Pity in this case, I'd be wrong.

Sleeping men grow beards faster than conscious men.

  1. Facial hair growth is proportional to testosterone levels

  2. Testosterone levels increase when you're asleep, peaking during REM sleep

If your agent has been asleep for 24 hours, and he was drugged 3 hours after he shaved, his beard will be slightly longer than it would normally be at 27 hours after a shave. (I'm guessing he'd say it felt more like a 36 hour beard, but it depends on how the knock-out drug worked. Did it put him to sleep? Did he REM?)

However, when he's knocked out (eg due to a concussion or injury), his Testosterone level will drop, by up to 70%. This can also occur with Alcohol or other things which knock you out chemically. He'll wake up 24 hours after being knocked on the head and guess from his beard he'd only been out for 12 hours.

He would be highly certain of his prediction - I reckon he could learn to predict it within an hour if he does know his body that well.

He'd be quite surprised at how wrong he would be in this case.

(He also could've been knocked out for months and the bad-guy shaves him the day before withdrawing the IV.)

  • $\begingroup$ He could easily know that beards grow faster in your sleep, precisely to avoid his getting it wrong for that reason. $\endgroup$
    – Mary
    Commented Sep 17, 2020 at 22:43
  • 1
    $\begingroup$ @Mary But was he asleep, unconscious, or a mix of the two? $\endgroup$
    – Ash
    Commented Sep 17, 2020 at 23:41

0-24 hours is easy; most men who shave can estimate within around 6 hours by feel.

1-3 days is pretty easy for me to determine number of days (but nothing more precise) by feel. After that, it's just "scruffy" and hard to gauge by feel.

For longer periods of time, Agent Jones needs to know his beard's average growth rate...and have a ruler handy...and perhaps should take an average of 10-or so hairs, so perhaps a bit of scratch paper.


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