Jaargetijde literally translates to "season" (as in, a season of the year). It's a more formal and poetic word for 'season' than the common 'seizoen', making it stand out.
In the song, Jan Smit uses it to describe his emotional state, singing "Voelt als een winters jaargetijde" (Feels like a winter season). This metaphor beautifully conveys a feeling of coldness, emptiness, and longing, making the word particularly evocative in this context.
Seven nights without sleep, and Jan Smit blames the girl who left. The title is a complaint with a joke buried in it: "Want hoe kan ik van je dromen / Als je mij niet slapen laat" ("Because how can I dream of you / If you won't let me sleep"). He wants her back the ordinary way, in a dream, and she has taken even that away by keeping him awake.
The verses stay on small physical details instead of big declarations. He lies there waiting for a falling star to wish on, he counts the days ("Het duurt nu zeven dagen lang", "It's been seven days now"), and the closest he gets to her is a phone call that turns out to have been a dream. The line doing the most work is the flattest one: "Met het bed onder m'n rug" ("With my back against the bed"), which is what hours of staring at the ceiling actually feel like. Then the bridge dismisses every other woman in four short lines and the chorus returns, still asking the same unanswerable question.