What Happens If You Leave Box Hair Dye In Overnight? Will It Ruin Your Hair?

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If you leave hair dye in your hair overnight, there’s a good chance it will damage your hair. However, this depends on the type of dye you’re using.

What happens if you leave box hair dye in overnight? You will wake up with dry, damaged, brittle hair. It’s not a good idea to ignore the directions for these products and wing it.

So before you rush out to the drugstore and begin a DIY color experiment, we are going to take a look at your hair, go over different hair dyes, how they work, what happens if you leave them in too long (or overnight).


Hair Science 101

First things first. In order to understand what happens if you leave box hair dye in overnight, you need to understand your hair structure and how dyes work and interact with your hair to color it.

A natural strand of hair is made up of two (or sometimes three) components.

1. The cuticle

The cuticle is the outermost, colorless layer. It’s made of flattened cells that overlap like the scales on a fish or clay tiles on a rooftop. The cuticle protects the inside of the hair shaft from damage.

2. The cortex

The Cortex is made up of long proteins that twist, like the old-school telephone cords. Those twists give the hair elasticity and allow it to stretch to a certain point, without breaking. The pigments that give your hair its natural color are found among these protein strands.

3. The medulla

The medulla is not present in every hair. It’s a soft, spongy tissue that is generally found in coarse, thick hair. Fine hair doesn’t tend to have a medulla. The medulla does not affect the color of your hair or how it behaves, so we don’t need to worry about it.

You’ll understand why this is important when we get to how different types of hair dye work.


Box Hair Dyes

There are different types of box hair dyes. Semi and demi-permanent (i.e. non-permanent hair dyes) and permanent hair dyes. They work to color your hair in different ways.

This also means that there’s a difference between what happens if you leave in non-permanent and permanent hair dye in overnight.

There are different types of box hair dyes. Semi and demi-permanent (i.e. non-permanent hair dyes) and permanent hair dyes. They work to color your hair in different ways.

Permanent Box Hair Dye

Permanent box hair dye color uses a developer to open the hair cuticle to allow the color to penetrate the hair shaft for longer-lasting color than a non-permanent dye.

The developer consists primarily of ammonia or hydrogen peroxide. 

Ammonia is an extremely harsh chemical that works by stripping the layers of the hair’s proteins so that the dye can be absorbed into the hair shaft. 

Hydrogen peroxide-based developers work by supplying the necessary oxygen gas to develop the color molecules and create a change in the natural hair color.

Although it’s less damaging than ammonia-based developers, hydrogen peroxide causes oxidative stress which ages your hair and is very drying. This is why post-color hydration is super important.

Non-Permanent Hair Dye

A non-permanent box hair dye just coats the exterior of the hair strand. It is not mixed with a developer that contains ammonia or peroxide that comes with permanent hair dye. A non-permanent colour will not lighten your hair – just add colour on top.

As the name suggests, semi and demi-permanent dyes will not last as long as permanent dye. Demi-permanent dye lasts about 30 washes and semi-permanent dyes last about 5 washes.

You cannot use a light semi or demi-permanent color over darker hair and expect to get the color on the box. They do not lift (lighten), they only deposit color. So for example, say you were going for purple and you have dark brown hair.

If this is the color on the box:

This is the color you’ll end up with if you have darker hair:

If you choose a lighter blonde color and try to apply it to brown hair, you will most likely not even see a difference. Non-permanent colors do not lighten your hair, they only deposit color on top.

You cannot use a light semi or demi-permanent color over darker hair and expect to get the color on the box. They do not lift (lighten), they only deposit color.

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How Long Should You Leave Box Hair Dye In?

Different box dyes will come with different wait times, but the wait times are generally 30 – 45 minutes.

Read the instructions!

These wait times for any particular product are there for a reason. It’s the time that it takes for the product to do its job.

Now, if you have particularly thick and/or low porosity hair, it may take a few minutes more, but not hours or overnight.

Yes, leaving hair dye in longer will make it darker. But it will lead to hair damage, too.


What Happens If You Leave Box Hair Dye In Overnight?

Hair dyes are formulated to do their thing and then stop working after their recommended wait time. So, your hair color will not be any difference between the color after the recommended 30-45 minute wait time in the instructions and the color when you wake up having left it in overnight.

But leaving it in overnight can cause several issues. If you are using a permanent dye that contains harsh developer chemicals, it will strip your hair of moisture which will cause it to become brittle and more prone to breakage and split ends.

If you are using a permanent dye that contains harsh developer chemicals, it will strip your hair of moisture which will cause it to become brittle and more prone to breakage and split ends.

It will also dry out your scalp.

Prolonged exposure of your scalp to harsh chemicals can cause irritation and sensitivity.

Regardless of what kind of dye you use, permanent or non-permanent, and no matter how carefully you try to wrap it up in a shower cap, plastic bag or an old towel, you will no doubt wake up to find your sheets and pillowcases stained.

Your hair will also be a dry gummy, sticky mess that will be MUCH harder to rinse out.

No one wants that!


Take Away

Leaving hair box dye in overnight will not change, darken or lighten your hair any more than the specified wait time on the product instructions.

All it will do is leave a mess and your hair in worse condition than you started out with.

Written by Kayla Young

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