Mystery #3Tier 2brain / body clock
Why do you feel sleepier and sleepier the longer you stay awake?
First thing in the morning you can spring out of bed, but by late evening your eyelids feel like they have weights on them. The longer you stay awake, the harder it gets to keep going, as if tiredness has been quietly stacking up all day. What exactly is piling up inside you?
Your guess
What do you think is stacking up the longer you stay awake?
There's no single magic word to type, have a go in your own words, then reveal the answer to see how close you got.
Make a guess to unlock the hints and the answer.