Lice eggs, commonly called nits, take about 6 to 9 days to hatch after being laid, with one week being the average. This timeline is consistent regardless of hair type or length, because what drives hatching is warmth from the scalp. Understanding this window matters for both identifying an active infestation and timing treatment correctly.
The Full Life Cycle After Hatching
Once a nit hatches, the tiny louse that emerges is called a nymph. Nymphs are about the size of a pinhead and look like smaller versions of adult lice. They need another seven days of feeding on the scalp before they mature into full-sized adults capable of laying their own eggs. Adult lice then live for roughly 30 days on a person’s head, during which time a female can lay several eggs per day.
That means from the moment an egg is laid to the point where the louse that hatched from it starts producing new eggs, you’re looking at roughly two to three weeks. This compressed timeline is why infestations can escalate quickly if left untreated.
How to Tell if a Nit Is Still Viable
Not every nit you find in hair is alive or waiting to hatch. The color and location on the hair shaft tell you a lot. Live, developing nits are white, yellow, beige, or pale brown, and they darken as they get closer to hatching. They’re about the size of a sesame seed, slightly oval, and glued tightly to the base of a hair shaft near the scalp. Unlike dandruff, you can’t easily flick them off.
Dead nits tend to look brown or black and may appear slightly deflated. Empty casings left behind after hatching turn white, gray, or translucent and are often found a quarter inch or more from the scalp, since the hair has grown out since the egg was originally laid.
This distance from the scalp is a practical shortcut for judging whether an infestation is still active. If the only nits you find are more than a quarter inch from the scalp and you see no live lice or nymphs, the infestation is likely old and no longer active.
Why Eggs Need Scalp Warmth to Hatch
Lice eggs depend on body heat to develop. The incubation time is directly tied to temperature, and eggs only hatch within a range of roughly 73°F to 100°F (23°C to 38°C). The closer an egg sits to the warm scalp, the more reliably it stays within that range. This is why female lice cement their eggs right at the base of the hair shaft rather than further out.
It also explains why nits that fall off the head are no threat. Any egg that detaches from the hair loses its heat source and will not hatch. Lice themselves can’t survive off a human body for more than about two days, so stray nits on pillows, hats, or furniture are essentially dead ends.
Why Treatment Requires a Second Round
The hatching timeline is the reason most lice treatments call for a second application 7 to 9 days after the first. Many over-the-counter treatments kill live lice effectively but don’t penetrate the eggshell well enough to kill developing nits. By waiting about a week, you allow any surviving eggs to hatch, then kill the newly emerged nymphs before they’re old enough to lay eggs of their own.
The goal is to treat again after all remaining eggs have hatched but before the new generation can reproduce. If you skip the second treatment or do it too early, you risk letting a few nymphs slip through and restart the cycle. If you wait too long, those nymphs may have already matured and begun laying fresh eggs.
Checking for Hatching Progress
If you’re monitoring an infestation during treatment, comb through wet hair with a fine-toothed nit comb every two to three days. Wet hair makes lice slower and easier to spot. Look for both live nymphs (tiny, moving, pinhead-sized) and the color changes in nits described above. Darkening nits suggest hatching is imminent, while pale or translucent casings mean those eggs have already hatched or are empty.
Squishing a nit between your fingernails can also help you assess it. A live egg will feel like it “pops” when crushed. A dead or empty one typically won’t give the same resistance. Combining this hands-on check with regular combing gives you the clearest picture of whether the infestation is resolving or persisting between treatment rounds.