Common, and usually ordinary neck painStrong evidence
Neck pain is common anyway: 30% to 50% of adults have it over a year, more often women, and most often in middle age. NHS inform names "being in the one position too long" among common causes of neck problems, and the NHS says don't keep your neck in the same position for a long time.
No study has measured neck pain in passengers after flights. In a lab study of an aircraft seat, discomfort in the upper body became pronounced within about 30 minutes, and a little recline and more legroom eased it; in 29 volunteers who sat for 3 hours in a mock-up cabin, the back of the neck and both shoulders were among the most uncomfortable areas.
Sources 1, 2, 3, 4, 5
Dozing upright: why the head dropsEmerging evidence
In 6 people napping for 4 hours, an upright seat gave 29% less total sleep and 79% less dream (REM) sleep than lying flat, and the authors suggest "it is difficult to maintain the head in a comfortable position for sleep when sitting upright", especially in REM sleep, when the muscles relax most. A seat reclined to 40 degrees gave about as much sleep as lying flat. So as you drop off upright, the head nods forward or tips to the side and the neck spends long spells at the end of its range (our reading). The NHS lists the neck becoming locked in an awkward position while sleeping among the commonest causes of neck pain. This was one very small sleep-lab study.
Sources 6, 3
Neck pillows and headrests: two tiny lab studiesEmerging evidence
No study has tested a travel pillow against neck pain or for sleep on a plane. In a lab study of people resting upright in an office chair, read only as an abstract, a pillow that supports the chin from the front steadied the head and neck more than no pillow. In a reclined business class seat, a headrest made no difference to neck muscle activity, though passengers expected more comfort with one; some of that study's authors work for a seat maker. These are two tiny lab studies.
In 30 young men, a small lower back support with the backrest slightly reclined brought the head back over the shoulders. So a pillow is a comfort aid: worth using if it stops your head dropping, not if it pushes your head forward (our reading).
Sources 7, 8, 9
Screens, turning and lifting bagsEmerging evidence
In 24 young adults in a mock-up cabin, using a phone bent the neck further and was less comfortable than resting, more so for games than reading, and a higher armrest eased it; using a laptop bent the upper back, neck and head further forward than watching the seat-back screen.
Cabin crew rated handling carry-on bags their hardest task on the shoulders, and the neck and shoulders were their commonest sore area; lifting cases to head height loaded the shoulders beyond UK manual handling guidance in a lab study. Neck pain was reported by 30% of 105 cabin crew in one survey and, in Israeli flight attendants, was linked with stress. These are small surveys of people who work in cabins, not passengers.
Sources 10, 11, 12, 13, 14, 15
Blood clots after a long flightStrong evidence
A long flight raises the risk of a blood clot in a leg (a deep vein thrombosis, DVT) or in the lungs (a pulmonary embolism) a little. In 8,755 employees of international organisations, there was one clot per 4,656 flights of over 4 hours, and the World Health Organization's (WHO's) research project put it at 1 in 6,000 in healthy people. The risk rises with longer and repeated flights, by 26% for every 2 hours in the air in a review of 14 studies, and is highest in the first 2 weeks. A severe clot in the lungs on arrival affected 1.5 to 4.8 per million passengers on the longest flights in Paris. The NHS lists a past clot, recent surgery, cancer, pregnancy or a recent birth, and contraception or HRT that contains oestrogen among the things that make a clot more likely. Our back pain after flying guide gives more detail.
Sources 16, 17, 18, 19, 20, 21
When a scan mattersStrong evidence
Not for neck pain after a flight. In 1,211 people aged 20 to 70 without neck symptoms, 87.6% had disc bulges on a magnetic resonance imaging (MRI) scan, including most people in their 20s, so a scan finding doesn't explain a stiff neck after travel. New weakness, clumsy hands or the injury warning signs need a doctor, who decides on a scan. Neck, jaw or arm pain with chest pain can be a heart attack, the NHS says, and needs hospital tests, not a neck scan.
Sources 22, 23
How it usually goesStrong evidence
The NHS says "Most neck pain only lasts a few weeks". NHS inform says you should see the biggest change in the first couple of weeks, and most problems should have improved within 8 weeks. Neck pain often comes back: between half and three quarters of people with neck pain report it again 1 to 5 years later, which is why the plan keeps going. No one can promise a date.
Sources 3, 2, 24