Lash biology and the growth cycle

An eyelash has two halves: the part you can see, and the part that does the work. The visible lash, the shaft, is made of keratin, the same structural protein as the hair on your scalp and the surface of your nails. By the time it clears the lid margin it is biologically inert: a fibre with no blood supply and no nerves, which is why a trim never hurts. Everything living sits below the surface, in the follicle.

The follicle is a tiny pocket in the skin of the eyelid, and at its base is the bulb, where cells divide and keratinise to build the shaft from the root up. This is the only place a lash actually grows. Eyelashes are terminal hairs, thick, pigmented and coarse, set in several rows along each lid, roughly five to six rows above and three to four below, with about 90 to 160 lashes on the upper lid and fewer, shorter ones beneath. Unlike a scalp follicle, a lash follicle has no arrector pili muscle to stand it on end, though it does sit beside small oil and sweat glands that keep the lash line lubricated.

The difference between the living follicle and the dead shaft is the single most useful idea on this page. Anything that changes how fast or how long a lash grows has to act on the follicle. Anything applied to the visible lash, mascara, conditioner, oil, is working on inert fibre, and can only change how that fibre looks and behaves. Hold onto that distinction as you read the rest.

Every lash moves through the same three-phase cycle as all body hair, just on a much tighter timetable. Anagen is the active growth phase, when the follicle builds the shaft. Catagen is a short transition, when growth stops and the follicle begins to shrink. Telogen is the resting phase, when the fully formed lash simply sits in place until it is shed and a new anagen lash pushes through behind it.

For eyelashes the numbers are modest. Anagen lasts only around four to six weeks; catagen is a brief transition of a couple of weeks; and telogen, the resting phase, can run for several months before the lash lets go. Because each lash keeps its own schedule rather than moving in step with its neighbours, your lash line looks stable even though individual lashes are constantly cycling. At any given moment only a minority are actively growing, a figure often cited as roughly 40 per cent of the upper lashes, and fewer of the lower, while the rest are resting or on their way out.

This is why lashes are short, and why they stay short no matter how patiently you wait. The length any hair can reach is set almost entirely by how long its anagen phase lasts, and a lash grows for weeks where a scalp hair grows for years, anywhere from two to seven. A scalp follicle has time to extrude a fibre many centimetres long; a lash follicle switches off after a short burst, then rests. Left completely alone, a lash reaches its genetically set length and no further. That ceiling is set by the cycle, which is precisely why the cycle, not a bottle, governs how long a lash can ever be.

Losing lashes is not a malfunction, it is the cycle doing its job. Most people shed a handful of eyelashes across both eyes each day as resting lashes reach the end of telogen and drop out, usually so quietly you only notice the one on your cheek or in the basin. A lash lost this way already has a replacement forming behind it, and regrowing a single lash typically takes a couple of months, sometimes longer. Finding the odd lash on a cotton pad is normal; so is the occasional small gap that fills itself in over a few weeks.

Loss worth a second look is different in pattern, not just in amount. Noticeable thinning across the whole lash line, bald patches, both lids emptying at once, or lashes coming away in clumps rather than one at a time can point to something beyond the ordinary cycle. The medical term for eyelash loss is madarosis, and it has many possible causes, among them chronic inflammation of the lid margin (blepharitis), the autoimmune condition alopecia areata, thyroid disorders, some medications, and the habit of pulling lashes out, known as trichotillomania. None of these are cosmetic problems, and none are ones a conditioning serum can address.

Most day-to-day lash damage is mechanical, and it happens to the shaft rather than the follicle. The most common culprit is simple tension. Extensions that are too long, too heavy, or bonded to more than one natural lash pull on the lashes carrying them, and sustained pulling can drag a lash out before its cycle is done, a form of traction stress the lash line does not tolerate well. Lash lifts or perms that are left on too long or applied too aggressively can over-process the fibre and leave it brittle, frizzy or snapped.

Everyday habits add up too. Sleeping in mascara stiffens the fibre so it fractures against the pillow; waterproof formulas gripped and scrubbed off drag lashes away with them; and a mascara wand kept long past its use-by becomes a home for bacteria near the eye. Crimping with an eyelash curler, especially after mascara, when lashes are already stiff, can crease or break the shaft. And rubbing your eyes, whether from allergies, tiredness or removing stubborn makeup, is one of the most reliable ways to lose lashes you would otherwise have kept. The theme across all of it is friction and force, which is why gentler handling protects lashes far more than any bottle can.

Here is where honesty matters most. A cosmetic conditioner is applied to the visible lash, which means it works on inert keratin, not on the follicle beneath the skin. It cannot lengthen the anagen phase, wake a resting follicle, or change how many lashes you grow, those are functions of biology, and in cosmetics they are off-limits both scientifically and legally. What conditioning ingredients can do is change the condition and behaviour of the fibre you already have.

That is a real and worthwhile job. Humectants such as Glycerin and Sodium Hyaluronate draw moisture into the lash so it stays supple rather than dry and snap-prone. Panthenol and film-forming hydrolysed proteins coat and smooth the cuticle, so light reflects more evenly and lashes look glossier and less frayed. Conditioning peptides are studied as cosmetic actives for the appearance of lash density, though, and this is the part most brands leave out, much of that evidence is supplier data generated on scalp hair, and independent, peer-reviewed evidence in eyelashes specifically is limited. A well-conditioned lash is more flexible, less prone to breakage, and simply looks fuller and healthier. None of that is the same as changing the growth cycle, and any product claiming to do both is overreaching.

One ingredient class genuinely does act on the biology: prostaglandin analogues, the molecules behind the fastest-acting lash serums, which are thought to prolong the anagen phase. They are also the class documented in ophthalmology literature to be associated with darkening of the iris and loss of orbital fat around the eye. That trade-off, a physiological result carrying a physiological risk, is exactly why RUSKI leaves them out, and why we would rather condition the look of your lashes than reach for the cycle that grows them.

Conditioning and gentle handling are the right response to ordinary wear. They are not the right response to lash loss with a cause, and it is worth knowing the difference. See a GP, dermatologist or eye-care professional if lashes thin or fall out suddenly, if you develop bald patches or lose lashes from both lids at once, or if lash loss arrives alongside redness, crusting, itching or flaking at the lid margin, which can signal blepharitis. Loss that comes with hair thinning elsewhere on the body, or that follows a new medication or a change in your health, also deserves a professional opinion rather than a cosmetic one.

Two more cases are worth naming plainly. If you find yourself pulling lashes out and struggling to stop, that is trichotillomania, and it is best supported by a health professional rather than a serum. And if your lashes are suffering because of extensions or lifts, the person to see first is a reputable lash technician who can assess the damage, lighten or remove the set, and give the natural lashes room to recover. A conditioning routine can support the look of lashes through that recovery, but the mechanical cause has to be addressed first.

To be clear about our own place in all this: RUSKI products are appearance-based lash care, not treatments for the growth cycle. We formulate to condition the lash you already have and to support the look of fuller, healthier lashes; we disclose every ingredient in full; and we hedge our language on purpose, because the biology on this page is the reason a conditioner and a growth claim are not the same thing.

If you want to understand how peptides are studied as cosmetic actives, and what that evidence does and doesn’t show, read our companion guide, Peptides for lashes. If you would rather see how these ingredients are put to work across the range, our products page lists each formula with its purpose and its evidence. Either way you will be reading the science first, which is exactly the order we think it should come in.