There are several stages of tiredness.
First is the mere yawn and lowered focus.
Soon after, people cackle with lunacy. Short pulses of laughter in excitement or humor disrupt the hushed buzz of others concentrating. Words slur and ideas grow more illogical. People laugh more than think, and solutions are far from excellent. Hunger takes over as bodies decide to rely on calories instead of sleep. Locations of work often change to supply stimulation to a tiring mind.
Then eyes fade darker, and the focus greater; efficiency takes control of the project. Solutions replace the whimsical, humorous ideas of earlier, but this is not to say that creativity vanishes: it is refined.