Lithium commonly causes tiredness, and for many people it is one of the first side effects they notice. Drowsiness, fatigue, and a general sense of feeling slowed down are reported frequently enough that researchers and clinicians treat them as expected rather than surprising. The reasons behind this fatigue are not limited to a single mechanism; lithium affects your body clock, your thyroid, your brain’s electrical activity, and your fluid balance, and each of these can independently leave you feeling drained. The good news is that most of these pathways are well understood and manageable.
How Lithium Shifts Your Body Clock
One of the most direct ways lithium affects your energy levels is by altering your circadian rhythm, the internal clock that tells your body when to be alert and when to wind down. In animal studies, lithium lengthens the circadian period by roughly one to two hours, meaning the internal “day” runs slower than a normal 24-hour cycle.1PLoS ONE. Lithium Impacts on the Amplitude and Period of the Molecular Circadian Clockwork That kind of shift is not trivial. It can make mornings feel harder, delay your natural alertness window, and produce a lingering grogginess that feels like jet lag without any travel.
At the molecular level, lithium changes the activity of clock genes in the brain, increasing the expression of some genes that drive circadian oscillation and decreasing others that help reset the clock each day.2PubMed Central. The impact of lithium on circadian rhythms and implications for bipolar disorder pharmacotherapy These changes have been observed in the striatum, a brain region heavily involved in motivation and reward. Altering clock gene timing there could partly explain why lithium fatigue sometimes feels less like physical exhaustion and more like a motivational flatness, a sense that your drive to get things done has been turned down a notch.
A randomized trial in people with bipolar disorder tracked this in real time using wrist-worn activity monitors. Within the first week on lithium, participants showed roughly a 19 percent reduction in their most active ten-hour period compared to those on placebo. By week four, that reduction deepened to about 31 percent, and the onset of the active period shifted almost 1.7 hours earlier.3eBioMedicine. Early effects of lithium on circadian rest-activity and variability in patients with bipolar disorder In other words, lithium does not just make you subjectively tired; it measurably lowers your daytime activity level and reshapes when that activity peaks. The effect appears quickly and continues to develop over the first month of treatment.
The Thyroid Connection
Lithium concentrates in the thyroid gland and interferes with how it produces hormones. When thyroid output drops, the result is hypothyroidism, and fatigue is its hallmark symptom. In a study of real-world psychiatric practice in Italy, transient thyroid dysfunction appeared in about 37 percent of patients on lithium. Most of these cases resolved on their own, but roughly 9 percent of patients needed thyroid replacement therapy.4PubMed Central. Insights into the Incidence, Course, and Management of Lithium-Induced Hypothyroidism in Real-World Psychiatric Practice in Italy That might sound like a small fraction, but if you are in that group, the fatigue can be profound and will not improve until thyroid levels are corrected.
The risk of thyroid trouble scales with dose. The same Italian study found that each 100 mg daily increase in lithium dose raised the odds of progressing to more significant thyroid dysfunction by about 27 percent, and each small increment in serum lithium level (0.1 mEq/L) raised the odds by about 29 percent.4PubMed Central. Insights into the Incidence, Course, and Management of Lithium-Induced Hypothyroidism in Real-World Psychiatric Practice in Italy This is important because it means a small dose increase that pushes your blood level up just slightly can meaningfully change your thyroid risk. If you have been fine for years on lithium and then start feeling inexplicably exhausted after a dose adjustment, a thyroid panel is the first thing worth checking.
Thyroid-related fatigue from lithium tends to creep in gradually. Unlike the clock-shifting drowsiness, which often starts within the first week, hypothyroidism may take months to develop. It also comes with other clues: weight gain, feeling cold easily, dry skin, and constipation. When these appear alongside fatigue, a blood test measuring TSH can quickly confirm or rule out thyroid involvement. Guidelines for patients on lithium recommend regular monitoring of TSH, serum calcium, sodium, and creatinine.5PubMed. What an endocrinologist should know for patients receiving lithium therapy
Brain Slowing and Cognitive Fog
Even when your thyroid is fine and your circadian rhythm has adjusted, lithium can still produce a mental sluggishness that people describe as cognitive fog. This is not just perception; lithium produces measurable changes in brain electrical activity. EEG studies consistently show that lithium enhances the strength of slow-wave brain oscillations, particularly in the delta and theta frequency bands, the patterns associated with drowsiness and light sleep.6PubMed Central. Brain oscillations in bipolar disorder and lithium-induced changes Pushing the brain’s electrical baseline toward slower rhythms while you are awake is essentially nudging it closer to a sleep-like state during the day.
Some of the earliest EEG research on lithium, dating back to the 1960s, described “profound changes in the form of diffuse slowing, widening of the frequency spectrum, potentiation and disorganization of background rhythm” across all patients studied.7Journal of Psychiatric Research. The clinical laboratory and electroencephalographic effects of lithium Those are technical descriptions of a brain that is running at a lower gear. Researchers at the time concluded that lithium’s effects on the central nervous system were both cortical and subcortical, meaning the drug does not just affect one brain region, it shifts electrical activity throughout the entire brain.
The subjective counterpart is what clinicians have long called “lithium dullness.” A meta-analysis of the available studies found that lithium produced small but real negative effects on verbal learning, memory, and creativity.8PubMed Central. Lithium side effects and toxicity: prevalence and management strategies – Section: Cognitive impairment Earlier clinical descriptions also noted that the cognitive effects followed a dose-response pattern: higher doses meant more dullness. For many people, this mental fog is inseparable from their experience of feeling tired. When your brain feels sluggish, it takes more effort to engage with tasks, and the result registers as exhaustion even when your body is not physically fatigued.
Dehydration and Fluid Loss
Lithium affects the kidneys in a way that can quietly erode your energy. It interferes with the kidney’s ability to concentrate urine, leading to excessive urination and thirst. This side effect is one of the most common, with rates reaching up to 70 percent in long-term patients.9PubMed Central. Lithium side effects and toxicity: prevalence and management strategies – Section: Polyuria/polydipsia When you are losing more water than usual and not replacing it fast enough, even mild dehydration can cause fatigue, headaches, and difficulty concentrating. Many people on lithium do not connect their tiredness to dehydration because they do not feel dramatically thirsty; they simply feel run down.
The fluid loss also has a feedback loop with lithium levels themselves. When you are dehydrated, lithium concentrations in the blood rise because there is less fluid volume to dilute the drug. Higher lithium levels then intensify the other fatigue-producing mechanisms described above: more circadian disruption, more thyroid stress, more EEG slowing. Staying well-hydrated is not just general health advice for lithium users; it is a specific countermeasure against a cascade of fatigue-related effects.
Lithium is also a known risk factor for elevated calcium levels, sometimes through its effects on the parathyroid gland.10PubMed Central. Lithium-induced hypercalcemia with normal parathyroid hormone: A case report Elevated calcium can produce its own set of fatigue-like symptoms, including weakness and mental fogginess, adding yet another metabolic pathway through which lithium can leave you feeling drained.
When Other Medications Make It Worse
Lithium is frequently prescribed alongside other psychiatric medications, and certain combinations can amplify fatigue significantly. When lithium is combined with second-generation antipsychotics, excessive sedation becomes one of the primary adverse effects. A naturalistic study in China found that the total incidence of adverse effects across these combinations ran above 60 percent, with specific pairings producing noticeably different profiles.11PubMed Central. Effects of Lithium Combined with Second-Generation Antipsychotics for the Treatment of Manic Episodes in Patients with Bipolar Disorder Certain antipsychotics already have strong sedative effects on their own, and adding lithium on top creates a combined sedation load that can be substantially greater than either drug alone.
If you are taking lithium alongside another psychiatric medication and find the fatigue unmanageable, it is worth asking your prescriber which drug is likely contributing most to the sedation. Sometimes switching one of the companion medications to a less sedating alternative can make a bigger difference than adjusting the lithium itself.
Practical Steps to Manage the Fatigue
The single most discussed adjustment in the clinical literature is dosing schedule. Some people take lithium once daily, usually at bedtime, while others split it into two or three doses throughout the day. If the fatigue is worst during a particular window, shifting more of the dose to nighttime seems intuitively helpful, and many clinicians recommend it. However, comparative studies have found no significant difference in side effects between single and multiple daily dosing regimens.12PubMed. Lithium treatments: single and multiple daily dosing A separate review reached the same conclusion, finding no meaningful differences in plasma lithium levels, adverse effects, or symptom recurrence between the two schedules.13PubMed. Optimal frequency of lithium administration in the treatment of bipolar disorder: clinical and dosing considerations
That does not mean a schedule change never helps an individual. It means the evidence does not consistently favor one approach over another across populations. If bedtime dosing allows you to sleep through the peak sedation and wake up feeling more alert, it is a reasonable strategy even if the studies are agnostic. Here are the approaches that have the most support:
- Thyroid testing: Get your TSH checked regularly, especially in the first year and after any dose change. If hypothyroidism is driving your fatigue, thyroid hormone replacement can resolve it without needing to change your lithium.
- Hydration: Drink enough water to keep up with lithium-induced fluid losses. This helps stabilize your lithium blood levels and prevents the dehydration-fatigue cycle.
- Dose optimization: Work with your prescriber to find the lowest effective dose. Since many of lithium’s side effects follow a dose-response pattern, even a small reduction can meaningfully reduce tiredness if your therapeutic level allows it.
- Timing: Try taking the full dose or the larger portion at bedtime to concentrate the peak sedation during sleep.
- Blood monitoring: Regular checks of serum lithium, calcium, sodium, and creatinine catch metabolic causes of fatigue before they become entrenched.5PubMed. What an endocrinologist should know for patients receiving lithium therapy
Does the Tiredness Get Better Over Time
The clinical trial data on activity levels paints a somewhat discouraging short-term picture. In the first four weeks of lithium treatment, daytime activity continued to decline rather than stabilize, dropping steadily from about a 19 percent reduction in week one to roughly a 31 percent reduction by week four.3eBioMedicine. Early effects of lithium on circadian rest-activity and variability in patients with bipolar disorder That suggests the body does not quickly adapt to lithium’s circadian effects in the first month.
Longer-term clinical experience, however, is somewhat more reassuring. Many patients and clinicians report that the most intense sedation and cognitive fog do ease after the first few months, particularly once lithium levels have stabilized and the body’s clock has had time to recalibrate. The thyroid-related fatigue is the exception: if it develops, it will not resolve on its own and requires treatment. And the cognitive dulling described in the literature tends to persist at a low level for as long as a person takes the drug, though it becomes less noticeable as people adjust their routines and expectations around it.
Patient surveys confirm that drowsiness, fatigue, and lethargy are among the side effects most commonly reported by people taking lithium, and that these effects can compromise quality of life enough to threaten treatment adherence.14Revista Latino-Americana de Enfermagem. Bipolar disorder and medication: adherence, patients’ knowledge and serum monitoring of lithium carbonate This is worth emphasizing because stopping lithium abruptly carries real risks for people with bipolar disorder. If the fatigue is making you consider stopping, talking to your prescriber about adjustments is always preferable to discontinuing on your own.
How Lithium Fatigue Compares to Other Mood Stabilizers
People sometimes wonder whether switching from lithium to a different mood stabilizer would eliminate the tiredness. The answer is complicated. A head-to-head comparison of patient-rated side effect burden between lithium and valproate, another commonly prescribed mood stabilizer, found no significant difference. Patients on lithium rated their overall burden at about 1.4 on a standardized scale, while valproate patients rated theirs at 1.2, a gap that was not statistically meaningful.15PubMed. A comparison of patient-rated burden and incidence of side-effects: lithium versus valproate Interestingly, patients on valproate actually reported more individual side effects on average than those on lithium, even though the overall subjective burden was similar.
Every mood stabilizer has its own side effect profile, and fatigue or sedation features in most of them to varying degrees. Valproate can cause its own kind of tiredness and cognitive slowing. Lamotrigine is generally considered less sedating, but it carries different risks. The decision to switch medications is never just about one side effect; it involves weighing efficacy, tolerability, and the specific symptom profile of your condition. The evidence suggests that lithium’s fatigue burden, while real and sometimes disruptive, is not dramatically worse than its main alternatives when patients rate the overall impact on their lives.
What Lithium Does to Brain Waves During Waking Hours
Beyond the broad strokes of feeling tired or foggy, the way lithium reshapes your brain’s electrical landscape has some interesting implications. The enhancement of slow-frequency oscillations is not random noise; it represents a genuine shift in how the brain processes information at rest. Delta and theta waves dominate during sleep and deep relaxation. Boosting their presence during waking hours means the brain is, in a measurable sense, hovering closer to a sleep-like state even when you are going about your day.6PubMed Central. Brain oscillations in bipolar disorder and lithium-induced changes
There is a correlation between serum lithium levels and the magnitude of these brain oscillation changes, which reinforces the dose-response relationship seen in other side effects. People with higher blood levels tend to show more pronounced slowing. This is one reason clinicians aim for the lowest therapeutic level that keeps mood symptoms in check; the cognitive and energy costs of running a higher level are not abstract, they are reflected in measurable changes to how the brain operates during every waking hour. For people who feel that lithium has subtly changed their mental texture in ways that go beyond simple drowsiness, the EEG data offers a physiological explanation. The drug is not just making you sleepy; it is genuinely altering the rhythmic foundation of your waking brain activity.