Most people can tell when they’re tired. We yawn a lot, we suffer from low energy, and somehow the seven snoozes on the morning alarm just don’t feel like enough.

But unless they tell you (and you believe them), it can be much harder to tell when those around you are struggling with lack of sleep – and it seems many are.

New research suggests that teenagers are getting up earlier and going to bed later than their forebears, with relatively few notching up the advised eight hours a night – thanks in part to modern lifestyles where we’re burning the candle at both ends.

Little sleep is sometimes worn as a badge of pride, particularly among students. It’s an endurance test to be conquered, or proof you’re working hard.

But skimping on shut-eye can have serious real-world consequences, from the minor (missing a meeting because you overslept) to the major (falling asleep at the wheel).

No one wants their loved ones operating at a fraction of their capacity, for the good of both health and happiness. These telltale signs will help you see whether someone – teen or not – needs a bit more kip.

1. Counting the hours

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The maths varies from person to person, but sleep is a numbers game at heart. The HSE advises each of us to get between seven to nine hours of sleep a night – adding that the best sleep comes from a regular bedtime schedule.

"Watching television or using devices such as a smartphone right before going to bed can result in poor sleep. You should also avoid smoking, or drink alcohol, tea or coffee at least 6 hours before going to bed."

So if your loved one is spending every night out and then gets up early for work or school, it will be having an adverse effect.

2. Poor cognitive function

Bill Clinton once said: "Every important mistake I’ve made in my life, I’ve made because I was too tired." True or not, he’s spot on that sleep deprivation leads to impaired memory, and sometimes drastically altered judgement.

During the night, we need "several cycles of deep and rapid eye movement (REM) sleep to maintain normal thinking and decision making," explains Dr John O’Reilly, consultant physician with sleep health company SleepHubs.

"During deep sleep the lymphatic system literally washes the brain clean of waste products which build up in the day, leading to progressive fatigue and sleepiness. REM sleep is also when important memories and learning are retained and consolidated into long-term memory."

If your teenager doesn’t seem quite at the races, sleep deprivation is probably the most likely reason why.

3. Skin problems

We’ve all been there. A few late nights preparing for something important – a speech, a presentation, your first day at a new job – and on the big day you wake up to find a great big spot desecrating your chin.

Your body uses the time you spend asleep to repair your body, and studies have shown skin recovery to be 30% more effective in people with consistent, high quality sleep patterns. They don’t call it ‘beauty sleep’ for nothing, and if someone is suffering an unusual breakout, they’re probably in need of some extra rest.

4. Eye bags

Eye bags can signal an allergic reaction, and are commonly a sign of ageing. For teenagers, the purple bags probably reveal they’ve been up all night playing Fortnite.

Since time immemorial, eye bags have been considered a reliable flag that someone has been burning the midnight oil. There’s one spanner in the works – eye bags can be hereditary, so some folk are simply born that way.

5. Grumpiness

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It sounds obvious, but if your loved one generally emerges from slumber like a bear with a migraine, bouncing off the door frame and crying out for caffeine, it probably means they need a bit more sleep.

Look particularly for a change – sleep deprived people tend to be ponderous and irritable in the mornings, and enjoy gradual improvement in mood through the day. So if they seem to be having a case of the Mondays every day, the problem probably rests with the night before.

6. Weight gain

Studies have shown that people getting less than seven hours of slumber can be at higher risk of becoming overweight or obese.

According to the NHS: "It’s believed to be because sleep-deprived people have reduced levels of leptin (the chemical that makes you feel full) and increased levels of ghrelin (the hunger-stimulating hormone)."

Sudden weight gain or unusual appetite could mean a need for more time in bed.