Head lice go through three life stages: egg (nit), nymph, and adult. The entire cycle from freshly laid egg to reproducing adult takes roughly two weeks, and an adult louse can live up to 30 days on a human scalp. Understanding each stage helps you figure out what you’re looking at on a child’s (or your own) head and why timing matters for treatment.
Stage 1: The Nit (Egg)
A female louse lays her eggs directly on individual hair strands, almost always within a quarter inch of the scalp. She picks this spot for a reason: the warmth from your skin keeps the eggs at the right temperature to develop. Nits found more than a quarter inch from the scalp are generally dead or already hatched, which is a useful detail when you’re trying to assess whether an infestation is active.
Each nit is tiny, roughly 0.8 mm by 0.3 mm, oval-shaped, and yellow to white in color. They’re often mistaken for dandruff, but there’s one reliable difference: nits don’t flick off easily. The female louse cements each egg to the hair shaft with a glue-like sheath made primarily of proteins and fatty acids. Research from Montclair State University found that this sheath is chemically similar to keratin, the protein your hair is made of. It also contains compounds that stiffen and harden the casing, which is why nits feel like they’re welded in place and resist ordinary shampoo and water.
Under normal scalp conditions, nits hatch in about 6 to 9 days, with one week being typical. The empty shell stays glued to the hair and grows out with it, which is why you can sometimes find old nit casings inches from the scalp long after an infestation has been treated.
Stage 2: The Nymph
When a nit hatches, what emerges is a nymph, a miniature version of an adult louse about the size of a pinhead. Nymphs look like adults but are smaller and not yet able to reproduce. They begin feeding on blood from the scalp almost immediately.
Over the next seven days, a nymph molts three times, shedding its outer shell each time to accommodate its growing body. Each molt produces a slightly larger nymph. After the third molt, roughly one week after hatching, the nymph has reached full adult size and is sexually mature. This seven-day window is important for treatment: if you kill all the adults but miss the nits, a new generation of nymphs will emerge within a week and start the cycle over. That’s why most lice treatments call for a second application 7 to 10 days after the first.
Stage 3: The Adult
An adult head louse is about the size of a sesame seed, has six legs tipped with small claws designed to grip hair shafts, and ranges in color from tan to grayish-white. On people with dark hair, adults often appear darker because the louse’s semi-translucent body takes on the color of the blood it feeds on.
Adults feed by piercing the scalp and drawing small amounts of blood several times a day. This feeding is what causes the itching most people associate with lice, though the itch is actually an allergic reaction to the louse’s saliva, not the bite itself. Some people don’t develop this reaction for weeks after the initial infestation, which is one reason lice can spread before anyone notices.
A female adult begins laying eggs soon after mating and can continue producing nits for the remainder of her life. Adults survive up to 30 days on a person’s head, giving a single female plenty of time to establish a large population. Without treatment, successive generations overlap, with nits, nymphs, and adults all present on the scalp at the same time.
How Long Lice Survive Off the Scalp
Lice are obligate parasites, meaning they depend entirely on human blood to survive. If a louse falls off someone’s head onto a pillow, hat, or couch cushion, it will die within about two days without a blood meal. This is a much shorter survival window than many people assume, and it’s the reason that household cleaning during an outbreak, while reasonable, doesn’t need to be extreme. Focusing on items that touched the head in the prior 48 hours (pillowcases, hats, hair ties) covers the realistic risk.
Nits that detach from the hair are even less of a concern. Away from the warmth of the scalp, they rarely hatch. They need a narrow temperature range to develop, and room temperature surfaces don’t provide it.
The Full Timeline at a Glance
- Day 0: Female louse cements a nit to a hair shaft near the scalp.
- Days 6 to 9: Nit hatches into a first-stage nymph.
- Days 9 to 16: Nymph feeds and molts three times over about seven days.
- Day 16 onward: Fully mature adult begins mating and laying eggs, living up to 30 days.
From egg to egg-laying adult, the whole process takes roughly two to three weeks. Because each generation overlaps with the next, an untreated infestation doesn’t resolve on its own. The population simply grows until something breaks the cycle.
Why the Life Cycle Matters for Treatment
Most lice treatments, whether over-the-counter or prescription, are effective at killing live nymphs and adults but have a harder time penetrating the protective sheath around nits. That protein-and-fat casing acts as a physical barrier. This is why a single treatment often isn’t enough. The standard approach is to treat once to kill everything that’s moving, then treat again about a week later to catch any nymphs that hatched from surviving nits in the interim.
Manual nit removal with a fine-toothed comb shortens the process because it physically removes eggs before they can hatch. Combing every two to three days for about two weeks covers the full hatching window and catches any stragglers. Combining combing with a topical treatment gives you the best chance of ending the cycle in a single round.