Sleep restriction

Why the sleep diary is the part that actually does the work

It looks like homework. It is the only thing standing between you and guessing — and your memory of last night is wrong in a specific, predictable direction.

August 26, 20264 min read

Last reviewed August 26, 2026

Your memory of last night is wrong, and it is wrong in a particular direction. People with insomnia consistently recall more time awake than a recording shows. That is not carelessness — lying awake is vivid and memorable in a way that sleeping is not.

Which is the whole reason the diary exists. Every decision in CBT-I is made from numbers, and if the numbers come from recollection they are numbers about how the nights felt.

What it is actually measuring

Five things, every morning, from memory, taking under a minute:

What time you got into bed
Roughly how long you took to fall asleep
Roughly how long you were awake during the night
What time you got out of bed
How the night felt

Estimates are expected. Nobody is timing anything, and precision is not what makes this work — the trend across a week is.

From those five, two numbers fall out:

Total sleep time — time in bed, minus falling asleep, minus time awake.

Sleep efficiency — total sleep divided by time in bed, as a percentage. Nine hours in bed and six hours asleep is about 67%. Consolidated sleep usually sits above 85%. That single number is what the sleep window is set from and adjusted by.

Nothing can be prescribed without it

Sleep restriction means narrowing time in bed to roughly what you are actually sleeping. That sentence contains a number nobody can guess. Set the window from a feeling and you either restrict far too hard — which is unpleasant and unsafe — or barely restrict at all, and nothing changes.

The same for widening it. The window opens up when efficiency holds high across a week. Without a week of entries there is nothing to hold high.

The reason it helps beyond the arithmetic

Something happens after two or three weeks of entries that has nothing to do with calculations.

A bad night feels total while you are inside it. Tomorrow is ruined, this is never getting better, last night was the worst yet. Then you look at the row and it was your third-best week, and last Tuesday — which you have completely forgotten — was fine.

Take the free assessment to see:

✓Your Insomnia Severity Index (ISI) score
✓A personalized 6-to-8-week CBT-I plan, built by a sleep physician
✓How your sleep improves, night by night

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That is not positive thinking. It is the correction of a genuinely inaccurate impression, and it takes some of the force out of the catastrophizing that keeps people awake in the first place. The 3am arithmetic is arousal, and a chart is one of the few things that argues with it credibly.

Why the diary rather than a watch

Wearables estimate sleep well enough for a general picture. Two reasons CBT-I is still built on the diary.

The research is on the diary — the Consensus Sleep Diary is what the trials used, so the thresholds and the protocols are calibrated to it.

And for some people, tracker data becomes its own problem. Waking up, checking a score, and feeling worse because the number disagrees with how you feel is a real pattern with a name in the literature. A diary asks how the night was; it does not grade you at breakfast.

In Somnera

The diary takes under a minute in the morning. Efficiency, total sleep and the trend across weeks are worked out for you, so you never do the arithmetic. There is a morning reminder if you want one, and it goes quiet on days you have already logged.

Once the program finishes you keep access to the material, and logging continues if you want it — that part is optional and separate.

If you do one thing

Start the diary before you start anything else. Two weeks of honest entries before a single change is what turns the rest of CBT-I from generic advice into something fitted to your actual nights.

What the diary feeds into is sleep restriction, and the rest of this section covers it.

Somnera is a self-guided education program built on CBT-I, founded and written by Dr. Camilo Ruiz, DO, FACOI, FAASM. It is not a diagnosis and does not replace care from your own clinician. Start with the free assessment.

Citations

Carney CE, et al. The Consensus Sleep Diary: standardizing prospective sleep self-monitoring. Sleep. 2012;35(2):287–302. doi:10.5665/sleep.1642
Spielman AJ, Saskin P, Thorpy MJ. Treatment of chronic insomnia by restriction of time in bed. Sleep. 1987;10(1):45–56.
Edinger JD, et al. Behavioral and psychological treatments for chronic insomnia disorder in adults: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2021;17(2):255–262. doi:10.5664/jcsm.8986

Frequently asked questions

When you got into bed, roughly how long you took to fall asleep, how long you were awake in the night, when you got up, and how it felt. Estimates are fine — precision is not the point, the trend is.

Total time asleep divided by total time in bed. Nine hours in bed and six asleep is about 67%. Consolidated sleep usually sits above 85%.

For total sleep, often. But CBT-I is built on the diary, which captures your experience of the night — and there is a real risk in fixating on device data that does not match how you feel.

One to two weeks before anything is set, and then throughout, because the sleep window is adjusted from what the last week showed.

Leave it blank and carry on. A gap costs almost nothing. Abandoning the diary because of a gap costs you the program.