Sunken Eyes..

If lifestyle factors such as diet, sleep, exercise are fine then it could be genetics or natural aging process.
You can use a suitable night time under eye cream, use sunscreen in daylight and make sure not to forgot in the eye area. These will take few weeks to show visible difference once you start using regularly. Also Spa or dermatologists offer under eye brightening treatments that can fight the aging quite a bit if that is your thing. I started to get them and I can see some difference as for past 1 year my sleep is all over the place.

But nothing beats a good under eye color corrector and under eye concealer . I use the NARS one and it helps me not feel conscious if I am feeling tired . Mascara also helps my eye look more awake . I use very minimal makeup and add these two for days I need extra oomph.

 

Most pictures of bright eyes are well-made-up "natural looking" eye make up & photoshop

But if you're looking at real people and comparing, then the remedy is to massage (check videos for the right directions) with the oil that suits you - coconut oil, olive oil, sesame oil (Dont use mustard oil) or probably aloe vera gel

Sleep, water, food of course you're saying are good enough, so you've already dismissed them as reasons !

Lot of people swear by under-eye-gel - but that would add yet another product to your shelves - but if that's your style, go ahead!

 

Causes for the sunken eyes could be indoor and outdoor habits apart from sleep quantum, diet, inadequate fluid intake.

In dark background looking at monitor of smart ph9ne or desk top for long hours to be avoided. If such work can’t be avoided, then brighten the surrounding while in such work would help diminish the tonal change of tender skin under the eyes.

Lying on mattress, Holding and looking at bright monitor of the smartphone close to eyes in dark, hastens up the pigments under tender skin of the eyes to turn dark.

Usage of Artificial aids in the long run might turn out to be disadvantageous. Crow feet can develop under the eyes. Preservatives added in eye cosmetics to keep the cream fresh and free from germs can cause cancer though the label say quantity used is as allowed by FDA and safe!

Regular splash with rose water or and anointing closed eyes with tepid coconut oil just before to bed in night might be good in enhancing lustre or gloss of eyes.

God Bless.

 

It can be due to allergy, hereditary factors plus aging, dehydration, lack of sleep, increase in screen time, vit deficiency, tiredness or underlying issues.
Make sure you drink lot of water, sleep well and reduce screentime( not easy if we work full-time using PC's or use mobile a lot). Use of eye creams/ gels, massage etc can also help. I read that caffein / soda etc can cause dehydration, if so, it can be one of the reason. To balance it, we need to drink more water.