困 = 囗 + 木: a tree 木 sealed inside the four-walled enclosure 囗 (wéi), the same square that frames 国, 园, and 围. The whole glyph is a compound ideograph, ⿴囗木, with the wall wrapped fully around the wood. A tree needs open ground and sky to spread its roots and crown. Walled in on every side, it cannot grow. That single image carries the entire word.

The Shuōwén Jiězì reads the seal form along these lines and glosses 困 as the dead, exhausted timber of an old house, wood that has reached the end of its use. From there the sense widens to anything pressed in and unable to move: a person hemmed by walls, an army ringed by the enemy, a household squeezed by want. The wall does not strike the tree. It simply holds it, and the holding is the hardship.

Concrete senses came first. 困 meant to besiege and to trap, to pin a thing in place. The abstract reading followed: hard-pressed, stuck in difficulty, worn down. The modern colloquial sense of sleepy is the latest layer. A tired body sags and stalls the way a fenced tree stalls, so 困 took on the heaviness of the eyelids that will not stay open.

Japanese keeps the character as 困る (komaru), to be troubled or at a loss, and 困難 (konnan), difficulty, both holding the original sense of being boxed into a corner with no easy way out.

In spoken Mandarin 困 stands alone as an adjective meaning sleepy: 我困了 (wǒ kùn le), I am getting drowsy. This is the most common everyday use of the bare character. The sense is the eyelid weight of a body that wants to stall, distinct from 累 (lèi), which is physical tiredness or exhaustion. You can be 累 without being 困, and 困 without being 累.

Picture a young tree planted in a courtyard, then the courtyard walled up on all four sides with no gate. That is the literal glyph: 木 inside 囗. The roots want to spread but hit brick; the branches reach up but find only a square of sky. The tree is alive and yet it cannot move. Hold that one image, a green thing pinned in a stone box, and you have the root of every meaning.

Walk it through the compounds. The wall as a problem you cannot get past is 困难. The wall as a whole trap you live inside is 困境. The wall as the squeeze of poverty is 贫困. The wall as an army's ring around a city is 围困. The character never changes shape; only what does the pressing.

The sleepy sense fits the same picture. When you are 困, your eyelids are the walls, sliding down and boxing in your sight until everything stalls. 我困了 (wǒ kùn le): the tree of you, hemmed by heavy lids, can no longer reach toward the day.