At the first hint of summer, while most of us are firing up the barbecue, Charlotte Pennington pulls down her blackout blinds to ensure her home is as dark as possible.

‘When I tell people I hate summer – and I mean really hate summer – they can’t believe it,’ says the 26-year-old debt specialist adviser. ‘But it’s a truly visceral, physical and mental hatred.’

For Charlotte, who lives in Lancashire, with partner Tyler, 29, the warmer, lighter months mean a deterioration in her mood and increased anxiety, leaving her feeling unsettled and unable to sleep. Sometimes it can be a struggle to leave the house. That’s because she suffers from reverse seasonal affective disorder (SAD).

While we may be familiar with winter SAD (low mood during dark winter months), there is also a summer version of the disorder where the reverse is true, with the light and heat of the season triggering mental health symptoms.

Around 3 per cent of Britons experience winter SAD each year, according to the Royal College of Psychiatrists. It is thought to be caused by reduced light levels in autumn and winter disrupting hormones that regulate mood (such as serotonin) and our sleep-wake cycle (melatonin).

‘Estimates suggest 80 to 90 per cent of cases follow this winter pattern,’ says Adriana Kober, a clinical psychologist at Priory Hospital Barnt Green near Birmingham. But one in ten with SAD have the summer variant. ‘Summer SAD is an important and sometimes overlooked variation,’ she adds.

Indeed, when Charlotte told her GP that she thought she had summer SAD, he dismissed her, saying the condition was only for people who suffered low mood in winter.

‘He refused to believe it was the other way, too,’ she says. She still has not had a formal diagnosis.

Yet summer SAD ‘is a recognised and treatable mood disorder influenced by complex biological and psychosocial factors’, says Adriana Kober.

Dr Jon Van Niekerk, chair of the faculty of general adult psychiatry at the Royal College of Psychiatrists, agrees, adding: ‘Although SAD is more commonly associated with darker wintry periods, a smaller number of people can experience symptoms in summer.

‘The exact reasons are not fully understood, but it may relate to disruption of sleep patterns, changes in circadian rhythms, heat, humidity and longer daylight hours affecting mood regulation in vulnerable individuals.’

For example, in some people, it is possible that excess light can result in the brain not making enough melatonin, causing insomnia and jet-lag-type symptoms.

Lack of melatonin can also affect mood. When your body does not produce enough, it can trigger a depressive illness or anxiety.

While symptoms of winter SAD typically are low energy, lack of motivation, increased sleep and appetite and weight gain, ‘summer SAD can present differently – with symptoms such as anxiety, irritability, insomnia, restlessness, reduced appetite, weight loss and heightened sensitivity to heat’, explains Adriana Kober.

The key to recognising summer SAD, she says, is that there is a clear change in mood, energy and sleep, with ‘symptoms that recur at the same time each year and interfere with daily life, work, relationships and wellbeing’.

Charlotte, who had been diagnosed with anxiety and depression aged 13, started noticing her symptoms had a seasonal pattern at 16. ‘My anxiety and depression suddenly got much worse from around March and April,’ she says. ‘While other friends loved the warm air and bright light, I’d feel overwhelming anxiety and never wanted to go out – sometimes I couldn’t go to college.’

She noticed things getting easier in October: ‘Entering autumn, the sun setting earlier, the colder air, all made me feel much better.

‘In winter, it’s like my whole body decompresses. I was suddenly happier in myself, and much happier to see other people and go out.’

While winter SAD is generally thought to be down to falling levels in light (and so can sometimes be treated effectively using very bright artificial light to mimic the effects of sunlight), it is not known exactly what triggers the summer version.

Increased light levels may be part of it – particularly contributing to disrupted sleep.

‘Light is the primary environmental cue that synchronises circadian rhythm [timing of our body clocks] and in turn regulates hormonal systems linked to mood stability,’ explains Adriana Kober.

‘So changes in daylight can affect sleep patterns, energy regulation and emotional balance. In particular, the longer daylight hours may suppress melatonin production later into the evening, potentially disrupting sleep cycles.’

Sleep is something Charlotte especially struggles with in summer. ‘It’s too bright, too light and far too late [before it gets dark],’ she says. She also wakes regularly during the night, ‘which has adverse effects on my mental health, increasing my anxiety levels significantly. Then I feel ill and tired the next day.

‘This goes on for months. It spikes my anxiety and makes going outside really difficult.’

Meanwhile, some researchers have suggested that summer SAD may be triggered, at least in part, by an immune response to higher pollen counts at this time of year. One 2019 study by the University of Maryland found a link between high pollen days and worsening mood in those with summer SAD.

Another theory is that sensitivity to too much sunlight increases some people’s stress hormone cortisol – and they don’t have sufficient darkness to relax.

Other possible causes of reverse sad include the fact that high temperatures can suppress thyroid function and related hormones, which tends to make us feel ‘slowed down’, reduces motivation and lowers mood.

In addition to these, Adriana Kober says there may be social factors too. ‘Travel, changes in routine and work patterns, financial pressures such as holidays, and increased social expectations can also contribute to SAD onset.

‘Summer is often associated with social activity, positivity and physical confidence,’ she adds.

‘For individuals experiencing low mood or anxiety, this contrast between internal experience and external expectation can intensify distress.’

She adds that existing mental health conditions are often involved: ‘Individuals with anxiety disorders may find seasonal changes – particularly busy summer schedules – heighten feelings of being overwhelmed.’

Those with ongoing depression may experience more severe symptoms during seasonal transitions. People living with bipolar disorder can be particularly sensitive to changes in daylight, with seasonal shifts potentially influencing mood stability.

Adriana Kober says: ‘SAD rarely exists in isolation; it frequently overlaps with other psychological vulnerabilities.’

When it comes to treatment, there isn’t yet a summer equivalent of the light therapy recommended for winter SAD – this is delivered through therapy lamps that emit bright light of at least 10,000 lux, designed to mimic natural daylight.

‘Used for 20 to 30 minutes each morning, these help regulate the body’s circadian rhythms and influence mood-related brain chemicals such as serotonin and melatonin, which can become disrupted during darker months,’ says Adriana Kober.

‘Clinical research suggests that, when used consistently and correctly, light therapy can be an effective treatment for many people with winter SAD, helping to improve mood, energy levels and sleep patterns.’

While similar environmental treatments have been suggested in theory for summer SAD, such as cooling therapies, these have not been tested in clinical trials.

‘Cognitive behavioural therapy [CBT] – a type of talking therapy – is one of the most effective treatments for seasonal depression, including summer SAD,’ says Adriana Kober.

‘Adapted CBT helps individuals identify unhelpful thought patterns, build behavioural strategies, regulate routines and develop coping skills.’

Research published in 2015 in the American Journal of Psychiatry followed 177 adults with seasonal affective disorder. It found that CBT tailored for SAD was associated with lower recurrence rates and better long-term outcomes than light therapy two years on.

Medication, particularly antidepressants, can also be effective for moderate to severe symptoms.

Charlotte now takes antidepressants year-round. ‘But I periodically adjust them, particularly when it’s getting warmer outside,’ she says.

She is also religious about pulling down her blackout blinds in the early evening. ‘We’ve kitted out the whole house,’ she says.

She also tries not to go out to socialise until after 9pm, when light levels are lower – a tip she read online – and wears tinted glasses during the day to limit light exposure.

Two months ago, she came across a Facebook group for people with summer SAD.

‘There were 800 people all in the same situation as me,’ she says. ‘Suddenly, I didn’t feel alone and we could share our feelings and anxieties. It is a real condition and it’s miserable.

‘There were people on there who, like me, longed for dark, grey days again.’