Sleep Star, otherwise known as "Nemuri Boshi", is a specially designed tactic devised by Usopp in collaboration with Tony Tony Chopper in the anime/manga series One Piece. This special attack just goes to show how unique and creative Usopp can get into a fight and how well the Straw Hat Pirates' sniper and doctor can really work together in perfect sync.
The Sleep Star is a pink pellet, designed to put targets to sleep. It shows the power of fusing the expertise of Usopp in making specialized ammunition and that of Chopper in the medical field, especially in pharmaceuticals. This combination by the two members showcases the power of teamwork and the variety of skills the Straw Hat Pirates possess.
On his part, Usopp uses a slingshot – commonly the advanced Kuro Kabuto – to fire the Sleep Star. Upon hitting something or exploding, this pellet fires off a sleeping gas that takes effect on those who are near it. This is useful in defeating enemies without killing them and when one needs to have an upper hand in more complicated situations in battle.
A notable evolution of the Sleep Star concept would be the "Midori Boshi: Nemuri Gusa" technique. Usopp fires this version of Pop Green, which sprouts into a flower when fired, releasing a sleeping gas. This somehow combines Usopp's post-time skip plant-based armory with the sleep-inducing properties of the original Sleep Star. This further displayed the innovation and adaptation of techniques by Usopp throughout the series.
Among the highlighted moments in One Piece, Sleep Star and its variants have been very useful. One of the most important applications was during the Dressrosa arc, in which a technique that made people fall asleep was essential to disable Sugar, a prominent member of Doflamingo's crew. This really was a crucial moment that turned tables in the fight, showing how non-lethal techniques in a fight are strategically important in certain complex situations.
This means that, despite the high effectiveness of Sleep Star, it really has its limits if used in a world like One Piece. High-endurance or poison-resistant characters will have limited effects from the sleeping gas. Dosage and physiology of the target system could also affect the technique's efficiency and therefore call for a sort of tactical consideration in the process.
Such is the concept of sleep-inducing abilities in One Piece, but not limited to Usopp's inventions. Speculations and theories are in order for Devil Fruits that would grant sleep-related powers; however, there is no canonical Sleep Sleep Fruit in the series as of yet. This larger scope of sleep-based abilities within the world of One Piece provides even further context for Usopp's creation, making it a non-Devil Fruit alternative to attaining similar effects.
The Sleep Star and its related techniques also respect a more general theme in One Piece: characters finding creative, nonlethal ways to transcend trials and tribulations. This is relevant to Usopp's character development: from a simple liar and coward to a resourceful and strategic fighter who turns the tides of battle with ingenuity rather than by raw power.
Within the One Piece world-building, the Sleep Star symbolizes a point at which technology, natural compounds, and strategic fighting converge. This further describes how characters not born with superhuman abilities can transform knowledge, creativity, and teamwork into powerful techniques. It is this facet of the series that speaks to a large section of its fan base: the eclectic ways in which strength and fighting ability are gathered in One Piece.
The popularity of the sleep-themed ideas in One Piece has also translated into merchandise and fan works. For example, a "Sweet Dreams" line of night lights featuring One Piece characters has been made, cashing in on the appeal of taking series characters in restful scenarios. While this doesn't directly link with the Sleep Star technique, it further shows the general interest that sleep-related concepts evoke among the One Piece fan base.