Why do you feel so dreadful after a long-haul flight?

The flight attendant hands me a folded napkin and asks: ‘Ice?’

It’s a simple question, but I can’t seem to work out what to say. ‘Good?’ I manage to muster.

Let me explain: I’m 17 hours into an 18½-hour direct flight – the longest in the world – non-stop from New York to Singapore. I haven’t seen daylight in 30 hours, or inhaled fresh air since 6am as I took my seat before take-off.

The journey begins in New York, crosses the American continent before flying over the Pacific Ocean and landing in Singapore almost 19 hours later

It’s now 3am the day after I set off – or at least, it is on the east coast of America. And it’s quite possible I’ve gone a little insane.

Sadly, I am not en route to a relaxing, exotic holiday. In fact, I’ve turned into a flying scientific guinea pig.

My 9,534-mile journey with Singapore Airlines aims to answer the question posed by many a traveller: why do we feel so dreadful after a long-haul flight?

It almost goes without saying that being encased in a tin can, hurtling at hundreds of miles an hour at an altitude of about 30,000ft, while breathing recycled air for a day or longer, is never going to make you feel your best. But what does this kind of travel actually do to your body? And could there be any long-term damage?

Eve was wearing technology that would measure her vital signs while on the epic flight 

My investigation follows the announcement last month that Australian airline Qantas will in 2023 push the boundaries even further, with a direct flight lasting 19 hours 30 minutes from London to Sydney.

A test flight left London on Thursday morning, landing Down Under on Friday at lunchtime. The Singapore Airlines flight, running since 2004, clocks in at just an hour shorter – so should give a fairly good indicator of the physiological impact.

Before boarding, I donned an array of wearable health trackers. The gadgets would measure, among other things, my heart rate, blood oxygen levels and blood pressure, and even my brain-waves, to monitor stress and alertness.

I had a blood glucose monitor, which I was hoping would help me understand why I always get ravenously hungry on flights, despite the fact that I barely move.

I downloaded tests that would check my reaction times – to see how the flight affected my ability to think clearly.

I would measure my ankle circumference, to see how swollen they became – a warning sign of a sluggish circulation.

And a pollution monitor would continuously analyse the quality of air around me – the test that would, it turned out, provide the most alarming results of all.

Surrounded by cabin air that could cause cancer

Given my passion for sitting down for long periods and eating continuously, I imagined I’d rather enjoy my ultra-long flight. But then I began my research. I was startled by reports that harmful pollutants – linked to asthma, chest infections and even cancer – could be lurking in the cabin.

Tristan Loraine, a former British Airways captain who campaigns for cleaner air on planes, says: ‘The air you breathe in on most aircraft is compressed via the engine. So it becomes contaminated with toxic compounds in engine oils.’

And Terry Tetley, Professor of Lung Cell Biology at Imperial College London, says: ‘The heating of engine fuel emits tiny, toxic substances – known as ultra-fine particles – and other chemicals which penetrate deep within the sensitive lung lining and are taken up by the bloodstream. They cause inflammation of the lungs and increase the risk of some cancers.’

In a recent investigation, Loraine used a detector to measure levels of these microscopic toxic compounds aboard several flights.

About halfway into the flight my heart rate was back down at 49. I’d been awake for 14 hours by this point and could barely keep my eyes open

Cabin air contained more than ten times the amount outside the traffic-congested Victoria Underground station in London. Some airlines have tried to improve cabin air quality. The new Boeing 787 Dreamliner and the aircraft I’m flying on, the Airbus A350, contain special technology that is supposed to filter out harmful particles.

But according to Dr Susan Michaelis, an aviation safety researcher from the University of Stirling, this solves only half of the problem.

‘Air is only filtered the second time it enters the cabin, so the toxic particles have already made their way around the cabin once,’ she says. ‘Ultra-fine vapours, emitted when fuel is burned, are in the air at the airport, and this can enter the cabin before take-off.’

My flight left Newark, New Jersey, at 9.45am Eastern Standard Time, which meant it was 10pm on the same day at my destination, in Singapore.

Singapore Airlines say their air filters filter out almost 100 per cent of toxic pollutants. The devices on board are claimed to be the same as those used in hospitals.

Before boarding, my pollutant detector was in the green – almost zero toxic compounds detected.

After take-off, levels climbed steadily and peaked halfway through the flight – a red circle, signifying worryingly high levels of pollution in the cabin air. They fell, but remained moderately high. Prof Tetley says: ‘Exposure like this might trigger temporary inflammation and irritation on the airways and lungs. But it is chronic exposure that causes damage to the body’s cells, increasing the risk of cancers and lung infections.’

Even more scary was that my detector also picked up frighteningly high levels of the radioactive gas radon – 100 times higher than it is in my London flat.

Found in cigarette smoke and emitted from certain rocks, the potent gas is a known cancer risk when inhaled over long periods.

My flight left Newark, New Jersey, at 9.45am Eastern Standard Time, which meant it was 10pm on the same day at my destination, in Singapore

Radon is, apparently, a big concern on ultra-long-haul flights, which tend to travel at a higher altitude – about 40,000ft, compared to 30,000ft on a normal trip between, say, London and New York.

The Airbus I am aboard reaches about 41,000ft.

Professor Michael Lockwood, Professor of Space Environment Physics from the University of Reading, says: ‘We’re very concerned about radioactive particles in the atmosphere at extremely high altitudes. These high-energy particles break off from rays outside the solar system and enter the cabin air supply.

‘They attack human cells and are known to cause cancers of the skin and lungs. While it is unlikely to cause harm in a single flight, it’s a concern for frequent flyers.

‘Roughly 12 return flights to the US from the UK gives the same radiation dose as a CT scan – so roughly 200 X-rays. That would be roughly your safe radiation allowance for the entire year.’

Heart rate and blood pressure dart about

Other health effects of flying are caused by disruption to our internal body clocks, or circadian rhythm. These patterns determine our sleep and wake cycle and are governed largely by light.

Natural light triggers an alert response, kicking the cardiovascular system, brain activity and metabolism into action. When darkness falls, hormones are released that slow down bodily processes. Disruption can affect everything from how we digest food, to our immunity, and has been linked to poor cardiovascular health. My resting heart rate is lower than average, but nonetheless healthy – 49 beats per minute.

Within 20 seconds of taking off, it increased to 60, perhaps due to stress. There’s a similar effect on another indicator of heart health – my blood pressure – which increased by roughly ten per cent.

In the following three hours, my heart rate climbed to 65 beats per minute and stayed there – even though the lights were dimmed to simulate the evening time at destination. Prof Foster says: ‘Your body was in day mode and fighting the chemicals emitted in response to dark to make you feel sleepy.’

He also notes that any stressful experience can commonly cause a fluctation in heart rate throughout the day.

About halfway into the flight my heart rate was back down at 49. I’d been awake for 14 hours by this point and could barely keep my eyes open.

When we reached 5.30am New York time, when my body would usually wake up, I saw another spike in heart rate – up to 53.

Landing saw it climb further, to 60 beats per minute. My blood pressure increased suddenly too, though it remained relatively stable, albeit higher than normal, throughout the flight. Prof Foster says: ‘Fluctuating heart rate and blood pressure is a risk for people with pre-existing heart conditions or those who must take medication in accordance with body clocks. It could put vulnerable people at higher risk of stroke.’

Perfect conditions for a blood clot

Flying increases the risk of a potentially fatal blood clot in the lower leg called deep vein thrombosis, or DVT. The telltale sign of a sluggish circulation is swollen ankles as fluid collects in the lower limbs.

Consultant vascular surgeon Professor Mark Whitely says: ‘DVT isn’t only a result of sitting down for a long time. Reduced oxygen causes fewer anti-clotting proteins to be produced. A lack of moisture in the cabin dehydrates blood, making it even thicker. Combine this with the lack of movement and it’s the perfect storm for a blood clot.’

my blood oxygen monitor showed gradually depleting levels. Before take-off, my blood was almost 99 per cent oxygenated. Four hours in, it dropped to 90

Although there was no change in ankle width, my blood oxygen monitor showed gradually depleting levels. Before take-off, my blood was almost 99 per cent oxygenated. Four hours in, it dropped to 90.

Towards the end of the flight, my blood oxygen levels gradually stabilised at 95 per cent.

Although our resident GP doctor Ellie Cannon is dubious about my results. ‘With a reading of 90 you’d be having an asthma attack or fainting. I suspect you were either very cold or there was a problem with the machine.’

Hunger pangs and blood sugar spikes

The circadian rhythm also determines how the body processes food. Prof Foster says: ‘At night the body is less able to clear sugar from your blood because the metabolism slows down in preparation for sleeping. In the morning, our cells convert food to energy much more efficiently.’

Hunger hormones also increase at night – so the evening is a prime time for snacking. ‘If you’re eating when the body is programmed to be sleeping, it won’t process the food effectively, causing more dramatic spikes and drops in blood sugar,’ warns Prof Foster. ‘Over time, this increases the risk of obesity and type 2 diabetes.’

I take my reading with a blood glucose monitor a few times before I get on my flight – on an empty stomach – and it reads 4.7. But on board, this ‘fasting’ blood sugar level rises to 5.3.

When I eat my in-flight meal of chicken and rice, it rises slightly, before settling back down at 5.3 – the upper end of normal levels

Why is it so high while in the air?

‘Exercise can help the body to burn up excess glucose,’ says Prof Foster. ‘And on a plane you’re just sitting in one position.’

Within a couple of hours, as I usually find on flights, I begin craving snacks. ‘Your body clock thinks it’s the evening and releases hunger hormones,’ explains Prof Foster. When I eat a bag of nuts just after landing, it is four hours before my blood sugar falls back to normal fasting levels. ‘It can take up to two weeks for your metabolism to adjust,’ Prof Foster says. ‘These spikes can be risky if you’re on the cusp of type 2 diabetes.’

Threat of reduced brain function 

The stress hormone cortisol naturally rises in the day and drops in the evening. It can also peak in response to feelings of anxiety or stress, triggering the ‘fight- or-flight’ response. I’m measuring memory and decision-making using a smartphone game that tests my reaction times. ‘Brain areas involved in learning, memory and emotions are impaired at night,’ says Prof Foster. ‘At 4am, our brain capacity is akin to that of being legally drunk.’

Brainwave sensors on my headband detect changes every hour. Just before take-off, my brain activity was ‘calm’, but within an hour my stress response was unusually active.

As for my reaction times – I begin with a score of 16 out of 22, and end scoring just four. I trudge off the plane bleary-eyed, irritated with the knowledge that this is not the end of my journey. In 15 hours I’ll be back at the airport to board my flight home to London, via Doha.

I trudge off the plane bleary-eyed, irritated with the knowledge that this is not the end of my journey. In 15 hours I’ll be back at the airport to board my flight home to London, via Doha

‘It takes the brain and the rest of the body roughly two weeks for the clock to reset,’ says Prof Foster.

‘Meanwhile, world leaders fly across the globe making crucial decisions while chronically jet-lagged. We need to start considering chronic jet lag as an international health problem.’

Thankfully, I didn’t have any Government meetings to attend after I landed. But I did have this article to write, and what would usually have taken two days ended up costing half my week as I struggled to stay awake. To add insult to injury, I feel a cold coming on. Given the toxic air that filled my lungs, I just hope that’s the worst of it.

In a statement, Singapore Airlines said: ‘Airbus has demonstrated during extensive flight testing that the air quality on board the A350 meets all airworthiness requirements and international standards. Generally, medical conditions don’t stop you from flying. However, our passengers are encouraged to ensure that they are medically fit before flying with us.’