Head lice (Pediculus humanus capitis) are small, wingless parasites that live exclusively on the human scalp and hair. As obligate ectoparasites, they must feed on human blood multiple times daily to survive. A common misconception suggests lice are nocturnal, becoming active only after dark. Their activity cycles, however, are driven by a constant biological requirement for food, not a preference for darkness. This article clarifies their actual feeding and movement patterns.
Head Lice Fundamentals
Head lice have a three-stage life cycle that occurs entirely on the human host. The first stage is the egg, or nit, which the female cements to the hair shaft, usually near the scalp. Nits require scalp warmth to incubate and hatch within six to nine days. The newly hatched nymph resembles a small, sexually immature adult. Nymphs mature into adults after three molting stages, a process taking about seven days. Adults live for approximately 30 days, remaining near the scalp for warmth and food access.
Continuous Feeding and Movement Patterns
Feeding Activity
Head lice are not nocturnal; their survival depends on constant, frequent blood meals, making them active throughout the 24-hour cycle. An adult louse must feed at least six times daily to survive and reproduce. This biological necessity overrides any circadian rhythm, requiring them to feed both day and night. Lice pierce the scalp using specialized mouthparts and inject saliva containing an anticoagulant. If removed from the host, an adult louse will die within one to two days from starvation and dehydration.
Movement
Movement is driven primarily by the need to find a suitable feeding site or to transfer to a new host, not by the time of day. These insects are wingless and cannot jump. They move by crawling across hair strands using clawed legs adapted for grasping the hair shaft. While they can move quickly, they prefer to remain still to conserve energy when not feeding.
Why Nighttime Itching Occurs
The perception that lice are more active at night relates to the host’s experience and biological factors. The intense itching (pruritus) is a delayed allergic reaction to the injected saliva, often taking several weeks after infestation to develop.
During the day, external stimuli like work or school often mask the sensation of itching. At night, however, external distractions decrease, allowing the body to focus more on internal sensations. This makes the subtle crawling or lingering itch much more noticeable.
Furthermore, the body’s natural physiology during sleep contributes to increased sensitivity. Levels of anti-inflammatory hormones, specifically corticosteroids, naturally decrease at night. This hormonal fluctuation exacerbates allergic and inflammatory responses, intensifying the feeling of itchiness. The combination of reduced distraction and altered physiological responses causes the itching to feel worst after dark.