Chronic insomnia: This is the most likely diagnosis given Carmen's 12-month history of trouble falling asleep and long awakenings throughout each night, along with a constant feeling of lethargy and low mood. Her Insomnia Severity Index (ISI) score of 20/28 further supports this diagnosis, indicating moderate to severe insomnia.

Circadian rhythm disorder: Although less likely than chronic insomnia, this could be considered due to Carmen's difficulties sleeping and her history of taking time off work, which might have disrupted her sleep-wake cycle.
Restless legs syndrome: This could be a contributing factor to her insomnia, especially if she experiences uncomfortable sensations in her legs at night, but there's no direct information provided to strongly support this diagnosis.
Chronic pain: While not directly mentioned, chronic pain could be a underlying issue contributing to her insomnia and lethargy, but without more information, it's difficult to assess its likelihood.

Sleep apnoea: Although less likely given the information provided, sleep apnoea is a critical diagnosis not to miss due to its potential for serious health consequences, including cardiovascular disease. However, typical symptoms such as loud snoring, observed pauses in breathing, or morning headaches are not mentioned.

Other sleep disorders: Such as narcolepsy, periodic limb movement disorder, or parasomnias, are less likely given the information provided and would typically present with more specific symptoms not mentioned in Carmen's case.
Underlying psychiatric conditions: While low mood is mentioned, suggesting possible depression or anxiety disorders, these are not exclusively sleep disorders but could significantly impact sleep quality. Further evaluation would be needed to determine their presence and impact.