In fact, a prison report by the Centre of Mental Health suggests that the uncertainties of the first two weeks after release from a long period of incarceration – or, in the UK case, lockdown – are the time of most heightened psychological risk.
None of which is to say there's nothing you can do to improve flagging energy levels or mood, says Shah. "I think about it like a phone – when your phone loses battery you don't just use it less, you actually think of ways to restore the charge, you plug it in."
Here, she and other experts identify treatable factors that may be contributing to your lockdown low, and some surprising ways to tackle them…
Circadian rhythm
As a student, you might have been able to lie in until noon, but as we age, it gets harder and harder to have a deep and restorative night of sleep. Part of this is due to physical changes: the eyes naturally yellow with age, blocking the amount of blue light that can reach your retina and regulate your body's internal clock. A 2011 Danish paper found that yellow lenses were "significantly associated with an increased risk of sleep disturbances", meaning that we find it harder to get to sleep, wake up more often in the night and feel less refreshed in the morning.
What to do
Get outside as much as you can, as early as you can, to increase your exposure to blue morning light. When you eat influences your circadian rhythm, too. Shah recommends consuming all food within a 10- to 12-hour window: "If you're eating late into the night your body's really not equipped to [digest] that food," she says, and inhibited from its vital overnight repair work. "You'll feel tired and angry the next day."
If you struggle to fit in enough sleep during the week, weekend lie-ins won't help you catch up: consistency is key, so a daily 15-minute nap in the afternoon is better, if you can fit one in.
Hormones
The fall in women's sex hormones around menopause can have complex effects on your energy. Oestrogen and progesterone affect brain function, as well as your reproductive cycle, so dwindling levels can cause mind fog and fatigue. The menopause can also put women at higher risk of depression, which can set off a downward energy spiral, while hot flushes also interfere with sleep.
Falling testosterone levels as men age can have similar effects, causing low energy levels and poor sleep.
What to do
If your sleep started suffering after menopause, talk to your doctor about hormone replacement therapy, says Dr Louise Newson, GP and founder of the Balance menopause tracking app.
"I see thousands of women in my clinic who have tried so many things, then after three months of HRT they finally sleep.
"Lifestyle is really important too, but you can't replace hormones with anything other than with HRT."
According to Cancer Research UK, HRT can increase the risk of breast, ovarian and womb cancer but the increased risk is small (particularly in comparison to the risk of being overweight, smoking or drinking too much). For many women the benefits of taking HRT outweigh the risks, so talk to your GP about your options.
Rachel Clarkson, a Harley Street dietician, believes diet can also help. Soya products like tofu and edamame contain phytoestrogens, a range of compounds that are similar to naturally occurring oestrogen. The evidence is mixed, but a meta-analysis from 2008 found that women who took an extract of soya compounds found some symptoms falling by up to 50 per cent.
For men, it's worth asking your GP to check your testosterone levels – you may be referred for male HRT if they are particularly low.
Diet
"With ageing we stop producing intrinsic factor [a digestive protein] in the stomach", says Clarkson, which affects the absorption of fatigue-fighting vitamin B12 from our food. Many of us use caffeine to raise energy levels, which is efficient in the short term, but has a half-life of about five hours, meaning that 50 per cent of the caffeine in the coffee you drink at 5pm will still be in your system at 10pm.
What to do
Pay attention to your B12 intake if you don't eat meat, dairy or eggs, and increase your intake of foods fortified with the vitamin, like plant-based milks and nutritional yeast. People who struggle to absorb B12 from food can get injections from the GP.
Clarkson uses DNA testing to work out patients' individual caffeine tolerances, which vary hugely. Half can't tolerate more than 200mg of caffeine a day – three or four cups of tea, or two small cups of coffee. Anything more will keep them up at night, leaving them exhausted in the long run.
Alcohol also disturbs your sleep and as you reach midlife, your ability to tolerate it might be slowed as your body produces fewer digestive enzymes, so pay particular care.
Exercise
Going for a run might be the last thing you feel like when you're already exhausted, but paradoxically, exercise does increase your energy levels. A 2008 American study found that they could be improved by a fifth in people who complained of chronic tiredness, with just three 20-minutes sessions a week.
What to do
Importantly, the study also found that high-intensity exercise was less effective than the exertion level of going for a steady walk. Fatigue dropped by 65 per cent in those doing lighter exercise, but only 49 per cent in the intense group.
This might not be a surprise for anyone who feels wiped out after a spin class.
"Some people find doing high-intensity workouts can produce too much cortisol, the stress hormone, which makes it harder to switch off and sleep", says Newson. Instead, think about "doing less HIIT and looking at more strengthening stuff like yoga or Pilates".
Medication
In 2017, almost half of Brits were taking a prescription medicine every day, with numbers increasing with age and fatigue a common side effect. A third of women going to their GP for symptoms of menopause have been offered antidepressants, according to a survey by What Women Want at Menopause, which can make fatigue and sleeplessness worse.
What to do
Never stop taking any medication without discussing with a doctor first. Stopping antidepressants immediately without tapering down your dose can have unpleasant and dangerous side-effects. If you think you may have been prescribed a medication incorrectly, or you're concerned that the side effects are impacting your energy levels, discuss it with your GP.
Stress
This past year, the usual stressors of career and family have been magnified by the pressures of working from home and home-schooling, leaving everyone feeling overtired, overworked and overstretched.
"Stress is a large part of why we feel so exhausted," says Shah.
"There is a direct pathway between stress and energy... when we are very stressed, we imbalance our hormones, we impact gut health and our immune system."
What to do
This can be one of the hardest things to tackle, especially if it's caused by responsibilities you simply can't get out of, but it's worth making sure you keep meeting up with friends: studies show that the socialising hormone oxytocin can lower both stress levels and blood pressure.
Bizarrely, in moments of high anxiety, you could even try chewing gum. A 2012 study in Cardiff found it reduced stress, anxiety and muscular tension in participants, and it has been included in US combat rations for these reasons since World War I.
