Impaired declarative memory consolidation during sleep in patients with primary insomnia: Influence of sleep architecture and nocturnal cortisol release
Backhaus J, Junghanns K, Born J, Hohaus K, Faasch F, Hohagen F;
Commented by , 30 Jan 2007
Aim of the study
Sleep is thought to have an important role in the consolidation of memory. This study investigated whether this function of sleep is impaired in patients with insomnia, and how it relates to sleep architecture and cortisol release.
Method
16 patients with primary insomnia (DSM-IV) and 13 age-matched healthy controls were tested. In the evening they performed tests of memory; the first was a list of words in pairs to be memorized and then recalled when the first word of the pair was shown, the second a mirror-tracing task.
They then slept in the laboratory overnight with polysomnography and blood was taken for plasma cortisol every 15 minutes through an indwelling cannula so that subjects were not awakened. The same 2 memory tests were repeated in the morning.
Results
Performance on the declarative memory task (word pairs) was similar in the evening in the two groups, but improved overnight only in the normal subjects, not in the insomnia patients. Procedural memory (mirror-tracing task) improved in both groups. Sleep architecture differed in the 2 groups in that insomnia patients had a less total sleep time, lower sleep efficiency and less slow wave sleep.
Word-pairs memory improvement correlated with amount of slow wave sleep in normal subjects and with amount of REM sleep in insomnia patients. There was no significant difference in cortisol profiles but there was a significant correlation in both groups between memory consolidation and low serum cortisol levels in the middle part of the night.
Dr Wilsons's and Prof Nutt's comments
Although patients with insomnia complain of memory problems, this is the first study to show differences in cognitive function in insomnia which are directly related to sleep – hippocampus-dependent memory consolidation - was impaired in the insomnia patients, who had reduced slow wave sleep.
The crucial significance for us and for other sleep researchers is the fact that here is a solid outcome which we could potentially use to measure meaningful improvement in daytime function in chronic insomnia after treatment intervention. Studies of effects on memory consolidation of drugs which improve sleep, particularly slow wave sleep, are now urgently called for as the next step in this research.
If memory consolidation improves along with slow wave sleep this would validate the present findings and point to an important target for future treatments. The findings of a relationship with REM sleep in the patients, and with low cortisol at a particular period of the night, also merit further study.
A pilot study by Nissen et al. (ref. 1) a few months ago also showed evidence of impaired sleep-related memory consolidation in insomnia, on a procedural task this time. However the control group in this study had very poor sleep in the laboratory, so the relationship with sleep stages was not explored.
References
1. Nissen C, Kloepfer C, Nofzinger EA, Feige B, Voderholzer U, Riemann D. Impaired sleep-related memory consolidation in primary insomnia--a pilot study. Sleep 2006; 29 (8); 1068-1073