The word insomnia itself simply means lack of sleep, so in that sense, many characters in books might experience insomnia - being unable to sleep well due to anxiety or excitement, for instance. But the diagnosis of insomnia refers to habitual sleeplessness, with diagnostic criteria usually specifying that the patient should have had difficulty sleeping for at least a month before it can be considered insomnia, and that it should be interfering with the person's ability to function normally (that is, if the person doesn't get much sleep but feels fine, they don't have insomnia - they might just need less sleep than the average person).
There are three ways insomnia can manifest, and patients may have one, two, or all three:
- Difficulty initiating asleep - It's considered "normal" to fall asleep within about 15 minutes of getting into bed. For people with insomnia, it can take many minutes or even hours to fall asleep.
- Difficulty maintaining sleep - Waking up multiple times in the night and/or waking up early (e.g., before the alarm) and being unable to go back to sleep.
- Restless or nonrestorative sleep - Feeling tired, even after getting sleep, likely because the person was unable to go through full sleep cycles and get sufficient amount of sleep in each stage.
A person may be unaware that they fell asleep, but one tell is that they may have sudden dream-like images - this may happen, especially in a person with insomnia, for a couple of reasons:
1) The lightest stage of sleep is very similar to the stage of sleep in which we dream. In fact, if you look at brain activity of a person in the lightest stage of sleep and in dreaming sleep (REM or rapid eye movement), they'll look surprisingly similar. (Fun fact, they'll also look surprisingly similar to brain activity of a person who is awake.)
2) People with insomnia likely have a deficit of REM sleep. When a person has a deficit of REM sleep, an interesting thing happens that doesn't happen for other sleep deficits: they'll go into REM sleep more quickly and spend more time there. This phenomenon is known as "REM rebound." A person with insomnia may nod off and immediately have dream-like imagery and experiences. This, in fact, is a great explanation for people who report hearing voices, as well as for supernatural experiences; it's no coincidence that people are more likely to report seeing ghosts at night. Auditory and visual hallucinations are very similar to dreams, and are likely the result of the same processes that give us dreams.
The big question, of course, is what causes insomnia. One cause is that a person may be predisposed (genetically) to poor sleep. For instance, I recently did 23andme, and one of the things they look at in their health analysis is whether a person has genetic indicators of being restless during sleep. It makes sense, then, that some people simply don't sleep as well as others for no reason beyond what's written in their genetic code.
Of course, a person may also be genetically predisposed to other conditions that impact sleep, such as depression and anxiety. Insomnia caused by one of these conditions usually occurs because a person is unable to "turn off his/her brain" to fall asleep; instead, they may lay awake worrying or ruminating. But the direction of causality could be flipped, with insomnia causing depression. Sleep is one of the times your body replenishes important neurotransmitters. If your body isn't able to carry out those processes normally done during sleep/rest, they'll experience deficits that could manifest in a variety of conditions.
Among women, hormones can exacerbate insomnia. Many women report having insomnia during their period. (It's likely that hormones affect men's sleep as well, but unlike women, fluctuations in men's hormones are less predictable.) This is more likely to occur among women who have insomnia the rest of the time; it may simply be more severe at certain times in a woman's cycle.
Insomnia is also a symptom of post-concussive syndrome; that is, a person who had a concussion may end up experiencing insomnia. For some, this is short-term until their brain heals. For others, this is a long-term/permanent condition as a result of a head injury. (There are other symptoms of post-concussive syndrome the person may have, such as depression and tinnitus - ringing in the ears.)
Lastly, insomnia may be behavioral. Lack of good sleep hygiene could lead to insomnia. And people who have programmed themselves to be awake at night or to wake up easily at night (e.g., they care for a relative with a chronic illness and have to be awakened multiple times at night) may also end up developing insomnia as a result. However, people who have been diagnosed with insomnia and are working to deal with it tend to have the best sleep hygiene: they avoid things like reading or watching TV in bed, and often won't even have these distractions in their bedroom; they have a standard bedtime routine; and they tend to very thoughtful about what they consume, especially caffeine, close to bedtime. So if you're writing a character who has insomnia, this is one characteristic you could give them: an almost obsessive attention to sleep hygiene.
People with insomnia are also more likely to experience an unbelievably terrifying sleep disturbance: sleep paralysis. I could write a whole post (or two!) about sleep paralysis, which also explains many supernatural experiences. In the meantime, there's a documentary about it available on Netflix. I see insomnia used all the time in books; I rarely see related sleep disturbances like sleep paralysis. So if you're writing a character with insomnia, you might consider adding something like sleep paralysis in as well.
Other quirks of people with insomnia:
- They may find it difficult to sleep when they're supposed to and difficult not to sleep when they aren't supposed to, such as during the day, while watching TV, etc.
- They tend to be better at remembering their dreams, because they often wake up after a dream, and have some time to think about/process it.
- They may find it difficult to differentiate dream from reality, not necessarily in the moment, but after the fact. That is, they may remember something later on and be unable to tell if that actually happened or they only dreamed about it.
- They may be hesitant to tell others they have insomnia because people (usually normal sleepers, who may have a bout of insomnia every so often) will respond with remedies they use when they're unable to sleep. As a person with insomnia since I was about 8 years old - which at its worst, results in me getting only 1-2 hours of sleep a night, and typically, results in me getting 5-6 hours of sleep - I have tried just about everything, and have heard it all, from the mundane to the bizarre to the borderline inappropriate (my favorite, and you're welcome to use it in a book: a Starbucks barista who told me I needed "nature's sleeping pill, which is more of an action," followed by a gesture to make it very clear he was talking about sex).
Sleep well, writers! And if you don't, use it for story inspiration.