Mood swings: the hormonal, metabolic and neurological causes

Most mood swings have an ordinary, fixable driver behind them: a blood-sugar dip after a fast-carb meal, a short night, too much caffeine or alcohol, or a stressful stretch wearing your nervous system thin. A smaller but important share are driven by hormones, by the thyroid, or by a genuine mood condition such as the bipolar spectrum, PMDD or the emotional dysregulation of ADHD. Day-to-day ups and downs are normal and human. The signal that something deserves assessment is not the swing itself but its pattern: how extreme, how long, how disruptive, and whether it follows a cycle.

Normal mood, or a problem?

Mood is meant to move. Feeling flat on a grey Monday, irritable when hungry, tearful when overtired or buoyant after good news are all signs of a working emotional system, not a faulty one. The brain regions that generate mood are constantly nudged by sleep, food, hormones, stress and circumstance, so a degree of fluctuation through a day or week is expected.

What distinguishes ordinary fluctuation from something worth assessing is pattern, not the presence of swings. Useful questions: are the shifts out of proportion to what triggered them, or apparently unprovoked? Do they last days or weeks rather than hours? Do they follow a cycle, such as the days before a period? And are they disrupting your work, relationships or sense of safety? If the honest answer is yes, that is information, not a verdict, and it is the threshold at which a GP conversation is reasonable. None of this is a character flaw, and most causes on this page are common and treatable.

Everyday drivers you can change

Before reaching for a diagnosis, it is worth ruling out the four reversible drivers that quietly destabilise mood in almost everyone.

Blood-sugar swings

The brain runs largely on glucose, so it is sensitive to sharp changes. The popular "sugar crash" story is oversimplified, but the kernel is real: a large, fast spike from refined carbohydrate can trigger an insulin response that overshoots, dropping glucose below baseline two to three hours later. In a study of over 1,000 people wearing continuous glucose monitors, the size of that dip predicted hunger and low energy better than the peak did.¹ Hunger itself reliably sours mood: an experience-sampling study found that hungrier people reported significantly more irritability and anger, the state we call "hangry".⁷ Our piece on blood sugar and mood goes deeper, but the practical fix is to anchor meals with protein and fibre and not to skip them.

Poor sleep

Short or broken sleep is one of the most powerful mood destabilisers known. A set of three meta-analyses pooling 154 studies found that sleep deprivation, restriction and fragmentation all reduced positive mood and increased anxiety symptoms.² The mechanism is partly neurological: after sleep loss the amygdala, which generates emotional reactions, becomes more reactive and less well regulated by the prefrontal cortex, so small frustrations land harder.⁸ If you feel "wired but tired" rather than simply sleepy, our note on the overactive nervous system may fit better.

Caffeine and alcohol

Both are mood-active drugs that give with one hand and take with the other. Caffeine has a half-life of roughly five hours but ranging from two to ten, so an afternoon coffee can still be active at bedtime; meta-analyses show evening caffeine measurably reduces sleep efficiency, feeding the poor-sleep loop above.⁹ Alcohol initially calms by boosting the inhibitory neurotransmitter GABA, but as it clears the brain rebounds into a mildly overexcited, glutamate-dominant state, which is why anxiety, irritability and low mood the morning after, sometimes called "hangxiety", are so common even after moderate drinking.¹⁰

Stress load

Sustained stress keeps the stress-hormone (HPA) axis and cortisol elevated, which narrows your tolerance for frustration and shortens your emotional fuse. This is normal physiology under abnormal demand, not weakness. The realistic goal is not zero stress but recovery: protected downtime, and removing one or two avoidable load sources rather than overhauling everything at once.

Hormonal and metabolic causes

When mood swings track a biological cycle or arrive with physical symptoms, hormones and the thyroid move up the list.

The menstrual cycle and PMDD

Mild premenstrual mood change is common. Premenstrual dysphoric disorder (PMDD) is its severe end: marked irritability, low mood, anxiety or emotional lability that appears in the week or so before a period and lifts within days of bleeding starting. It affects roughly 1.8% to 5.8% of menstruating women in any given year.³ It is not "bad PMS" or oversensitivity: research suggests an abnormal central response to the normal hormonal shifts of the luteal phase, not abnormal hormone levels. Diagnosis depends on tracking symptoms prospectively across at least two cycles, because the defining feature is the timing.⁴

Perimenopause

The years before periods stop are characterised by erratic, swinging oestrogen rather than a smooth decline, and that instability matters more than the absolute level. Women in the menopause transition are about twice as likely to develop significant depressive symptoms as before it, and studies find mood tracks the variability of estradiol independently of how high or low it is, because oestrogen modulates serotonin and dopamine signalling.¹¹ New irritability, tearfulness or low mood in your 40s, especially with disrupted sleep or hot flushes, can be hormonally driven.

Thyroid

The thyroid sets the body's metabolic pace, and getting it wrong destabilises mood in two directions. An underactive thyroid (hypothyroidism) tends toward low mood, fatigue, sluggishness and emotional flatness, while an overactive thyroid (hyperthyroidism), as in Graves' disease, drives anxiety, restlessness, irritability and jittery mood swings that can mimic a primary anxiety disorder.¹² Because the symptoms overlap so closely with mood conditions, a simple TSH blood test is a reasonable early step for new, unexplained mood disturbance; our UK thyroid panel guide explains the numbers.

Testosterone in men

Low testosterone can contribute to irritability, low mood, fatigue and lost drive in men, though the link is easy to overstate. A meta-analysis of 27 randomised trials in 1,890 men found testosterone treatment improved depressive symptoms overall, but the benefit was concentrated in men who were genuinely hypogonadal and at higher doses, with little effect when levels were normal.¹³ Treatment helps a true deficiency, not low mood with normal hormones, which is the marketing trap behind much private "low T" prescribing.

When it points to something clinical

Bigger, more sustained mood instability, especially swings that are extreme, last days to weeks, or are unprovoked, can reflect a mood condition that deserves assessment. These are common and treatable, and naming them is the first step to help, not a label to fear.

The bipolar spectrum

Bipolar disorder involves distinct episodes of depression and of abnormally elevated or irritable mood with high energy. It is genuinely under-recognised: lifetime prevalence is around 2%, but UK patients wait on average roughly 9.5 years from first symptoms to diagnosis, and most are diagnosed with depression first because they seek help in the low phases.⁵ This matters because antidepressants alone can be unhelpful in bipolar disorder. If your swings include spells of needing little sleep yet feeling energetic, racing thoughts, or out-of-character spending or risk-taking, mention these specifically to a GP.

Cyclothymia

Cyclothymia is a chronic, milder relative of bipolar disorder: ups and downs that never quite reach the threshold for full episodes but persist for years and rarely settle. By definition the pattern lasts at least two years, is present more days than not, and never clears for more than two months at a stretch.¹⁵ People often describe themselves as simply "moody", which is exactly why it is missed.

ADHD emotional dysregulation

Difficulty regulating emotion is one of the most common and under-discussed features of ADHD, affecting between 30% and 70% of adults with the condition.⁶ Emotions arrive fast and intensely and can feel overwhelming, particularly around perceived rejection or criticism, a pattern sometimes called rejection-sensitive dysphoria. It is not a formal diagnosis in its own right, but it is a recognised, treatable part of ADHD. The clue is that the reactivity is lifelong and sits alongside inattention, restlessness or impulsivity.

Stabilisers with real evidence

Whatever the cause, several measures genuinely steady mood and are worth doing while you work out what is going on. None replace assessment for the conditions above.

• Protect sleep first. It is the single highest-yield lever, given how strongly sleep loss degrades mood.² Aim for a consistent schedule and 7 to 9 hours.
• Move regularly. A 2024 network meta-analysis in The BMJ pooling 218 trials and over 14,000 people found exercise meaningfully reduced depression, with walking or jogging, yoga and strength training among the most effective.¹⁶ Frequency matters more than intensity.
• Steady your fuel. Regular meals built around protein and fibre blunt the glucose dips that feed irritability and hunger.¹
• Audit caffeine and alcohol. Cut caffeine after early afternoon and review alcohol; both effects are reversible within days to weeks.⁹¹⁰
• Track it. A simple daily mood and sleep diary, plus where you are in your cycle, turns a vague "I'm all over the place" into a pattern a clinician can act on, and is essential for diagnosing PMDD.⁴

If you are tempted to add supplements for mood, our stack builder helps you avoid paying for overlapping or unproven products before the basics are in place.

Getting help in the UK

If mood swings are affecting your daily life, relationships or work, that is reason enough to seek help, and effective help exists. Start with your GP, who can check your thyroid, ask about your cycle, screen for ADHD and bipolar patterns, and refer onward where needed. If you ever feel unable to keep yourself safe, you do not need to wait: call NHS 111 and select the mental health option for urgent support, or call 999 in an emergency or if life is at risk. Samaritans are free to call any time on 116 123. Our start here guide can help you change one thing at a time rather than everything at once.

This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.