Researchers gain new evidence that local brain clocks help control sleep and wakefulness
All animals, from insects to humans, have internal circadian clocks that track daily light-dark cycles and help coordinate when the body sleeps and when it is active. A master clock in the brain’s suprachiasmatic nucleus (SCN) is known to synchronise many of these rhythms, but until now direct evidence for independent, functional local clocks inside other brain regions has been limited.
In a new study using mice, teams led by Professors Bill Wisden and Nick Franks at Imperial College London together with Dr. Mick Hastings’ group at the MRC Laboratory of Molecular Biology investigated a local clock in the tuberomamillary nucleus (TMN). The TMN contains histaminergic neurons that are silent during sleep and that release histamine during wakefulness to promote arousal. The researchers explored how disrupting a central clock gene in these cells affects sleep architecture, activity and memory.
Specifically, the team used genetic techniques to delete the core clock gene Bmal1 (also called Arntl or Mop3) selectively from histaminergic neurons in the TMN. They then compared the sleep–wake behaviour and brain activity of these Bmal1-deficient mice with genetically-matched control animals using electroencephalography (EEG) and behavioural tests.
The results showed that removing BMAL1 from histaminergic neurons disrupted the normal daily rhythm of histidine decarboxylase (HDC), the enzyme responsible for producing histamine. Mice lacking Bmal1 in these cells produced more HDC and had elevated brain histamine at times of day when levels are normally lower. This biochemical change translated into clear changes in behaviour and sleep:
- The animals were significantly more active and showed heightened arousal, with more prolonged periods of wakefulness.
- Sleep became fragmented and shallower: non-rapid eye movement (NREM) sleep exhibited lower delta power, indicating reduced sleep depth, and transitions from NREM to REM sleep increased.
- After enforced wakefulness, Bmal1-deficient mice recovered poorly: following five hours of sleep deprivation in an enriched cage, their recovery sleep was much shorter—around six hours less—than control mice, likely because HDC and histamine levels remained abnormally high and sustained wakefulness.
- Memory was also affected. In an object recognition test, mice without BMAL1 in histaminergic cells failed to distinguish between new and familiar objects, suggesting impaired memory consolidation linked to disrupted sleep.
Importantly, deleting BMAL1 in histaminergic neurons altered sleep architecture without abolishing the overall circadian rhythm driven by the SCN. This supports a model in which the master SCN clock sets the broad timing of rest and activity, while local, BMAL1-dependent clocks in specific neuronal populations fine-tune the balance between sleep and wake within that daily cycle.
Senior researcher Professor Bill Wisden, Department of Life Sciences, Imperial College London, emphasised the potential clinical relevance: “Good, consolidated sleep is essential for mental and physical health. Our findings suggest that local clocks in the brain are key contributors to the mechanisms that let us fall asleep, stay asleep and recover after sleep loss. Understanding these local clocks may open new avenues to improve sleep quality.”
Lead author Dr. Xiao Yu, also at Imperial College London, added: “This work provides strong evidence that the SCN is not acting alone. Local clocks in wake-promoting neurons help to shape when the brain promotes arousal and when it permits restorative sleep.”
The work was supported by the Medical Research Council, the Biotechnology & Biological Sciences Research Council (BBSRC), and the UK-China Scholarships for Excellence / China Scholarship scheme. Contact names associated with the press materials are Sam Wong and Gail Wilson at Imperial College London. The experimental study and its full data are reported in the journal Current Biology in the paper titled “Circadian factor Bmal1 in histaminergic neurons regulates sleep architecture” by S. Xiao Yu et al., published online November 13, 2014 (doi:10.1016/j.cub.2014.10.019).
Local circadian clocks allow tissues and cells to anticipate daily environmental changes. While the SCN functions as the master pacemaker, clocks are expressed widely across the body and in the brain. This study tested the role of a putative local clock within TMN histaminergic neurons in regulating the sleep–wake cycle. Histaminergic neurons are normally silent in sleep and activate after waking. The study found that hdc expression varies across the day and that deleting Bmal1 specifically in these cells abolishes that variation, elevating HDC expression and brain histamine during the rest phase. The resulting phenotype includes fragmented and shallower sleep, prolonged wakefulness at night, impaired recovery sleep after deprivation, and deficits in memory, while leaving core circadian rhythms intact. The data indicate that BMAL1 in histaminergic neurons times histamine biosynthesis to balance sleep and wakefulness within the overall SCN-driven daily cycle.
This article reports findings from controlled laboratory research in mice and does not imply immediate clinical application in humans. Further research is underway to determine how the SCN communicates with local brain clocks and whether those pathways can be targeted to improve sleep quality in people.