If you define insomnia as the inability to fall asleep, then hyperthyroidism fits that description better than hypothyroidism. A normal body has a circadian rhythm, and as bedtime approaches, body temperature drops, breathing slows, and the person becomes tired. When someone is hyperthyroid, body temperature, heart rate, and breathing are always elevated, because there’s too much thyroid hormone. When I was hyperthyroid, I had difficulty falling asleep. It felt like I was running when laying down—my heart was still beating over 100 bpm, so it was difficult to reach that relaxed state and fall asleep.
If you define insomnia as the inability to stay asleep, or to get deep, restful sleep, then hypothyroidism is one cause. In general, hypothyroid individuals tend to be fatigued and to sleep longer than others; however, they complain that it’s not restful sleep, and they wake up still tired or unrefreshed. This has to do with their inability to reach the deeper stage 3 and 4 levels of sleep. Here’s an interesting study about that (emphasis mine):
“All night EEG sleep patterns were recorded for 3 consecutive nights in 7 hypothyroid patients and compared to those of normal subjects of the same ages. No differences were noted between the patients and controls in the percentage for rapid eye movement (REM) sleep, total sleep time, time required to fall asleep, intervals between REM periods and number of REM periods for a given night. The most consistent and striking finding was that the percentage of time spent in stages 3 and 4 sleep was markedly reduced in all hypothyroid patients. The young adult hypothyroid patients showed a significant decrease in stage 4 sleep compared to normal young adult subjects, whereas the elderly hypothyroid patients had a significant decrease in stage 3 sleep compared to normal elderly subjects. Four of the 7 patients were again studied after several months of therapy with desiccated thyroid when they were euthyroid. After treatment, the percentage of time spent in stages 3 and 4 increased for each patient so that the levels for these sleep stages were no longer significantly lower than those of the controls. Also, after treatment the waking alpha EEG frequency increased slightly for each subject and there was improvement in psychological test performance. The findings indicate that, in hypothyroidism, stages 3 and 4 sleep are significantly decreased. The results also are suggestive that treatment with desiccated thyroid may produce an increase in these sleep phases. Auditory awakening thresholds for these sleep stages are higher than in other sleep periods, so that by this criterion they are “deeper” stages of sleep. It may be that a reduction in stages 3 and 4 sleep directly relates to patients’ feeling that they have slept little and/or poorly.” From:
Sleep apnea may be another reason hypothyroid patients feel unrefreshed in the morning. In one sleep study, treatment with levothyroxine significantly reduced the number of apnea episodes per hour:
“To determine the incidence and frequency of sleep apnea in persons with hypothyroidism, 11 consecutive patients with newly diagnosed disease were studied before and during thyroid hormone replacement therapy. Nine patients had episodes of apnea, with the number of episodes per hour of sleep ranging from 17 to 176 (mean, 71.8). Six of the nine patients were obese and had 99.5 episodes per hour compared with 16.3 episodes per hour in the 3 nonobese patients (p < 0.02). After 3 to 12 months of thyroxine replacement therapy, mean apnea frequency decreased from 71.8 ± 18.0 (SE) to 12.7 ± 6.1 episodes per hour, without reduction in body weight. There were fewer changes in sleep stage per hour during treatment (22.1 ± 4.9) than pretreatment (57.6 ± 14.5). Carbon dioxide response tests done under non-loaded and flow-resistive loaded conditions before and during thyroxine replacement therapy showed increases in the loaded respiratory effort and ventilation during thyroxine treatment. Sleep apnea episodes are common in persons with untreated hypothyroidism, even with normal lung function. Thyroxine replacement therapy decreases apnea frequency, even without change in body weight.” From:
So to answer your question, yes, hypothyroidism can cause poor, unrefreshing sleep, and combined with frequent waking from apnea episodes, you will feel like you haven’t slept much at all. If you’re already on thyroid medication, and still have unrestful sleep, then I’d be inclined to think you’re undermedicated. You may need to add some T3 to your dose, either directly, or in the form of desiccated thyroid. You can have a normal TSH and still be hypothyroid—that’s a problem all hypothyroid patients deal with.