Brain and sensory functions that facilitate sleep.

We received a question recently from a therapist:

Health visitors have been asking about the impact of babies being placed in continuously vibrating cots, prams, child seats and the impact these might have on the child’s sensory development. Do you have any advice?

There are currently a number of mechanical means of providing vibrating, rocking, swaddling and auditory sensory stimuli for the purpose of settling baby and supporting sleep and therefore parent freedom to engage in tasks/activities including sleep, themselves. What started out as a fairly straight forward question had us doing something of a deep dive into the world of sleep, and the more you delve the more nuanced and caveated the responses need to be! References looked at are at the end of the article.

Sleep is a common stressor for parents. In humans sleep patterns mature slowly and do not reach adult sleep patterns for many months or even years. Initially we sleep more than we are awake, not in one convenient chunk but rather in numerous short stints. As an adult we expect to have long wake periods in the day when we get tasks done and then recuperate in another long uninterrupted sleep period at night.

This mismatch is a huge demand and is why we are not designed to care for our babies on our own but through joint-parenting, grand-parenting, and support from others in our social group (alloparenting). This way parents are able to get the sleep they need, and babies can take the time they need to mature their sleep-wake cycles without the stressors of unrealistic social expectations and resultant parental exhaustion and stress.

We see sleep as a problem to be solved. In the same way we have invented machines to take over most of our daily tasks, can a mechanical means of enhanced and repetitive sensory stimuli be the answer we crave or could this impact sensory development and create future challenges? We think the answer is…we don’t yet know!

It turns out for hundreds of years we have been inventing ways to help babies to sleep without the need to carry them so we can get on with daily tasks. This article in the Smithsonian magazine gives a great overview of the history of cribs and other inventions to support sleep away from a parent. 

Having done a bit of exploring into the still somewhat mysterious world of sleep, we will share something of what we have learnt.

Baby sleeping next to parent.

WHAT WE KNOW ABOUT SLEEP:

  • We spend approximately a third of our lives asleep – it’s a vital occupation for physical and emotional development and wellbeing, also for brain development. It’s when neural connections form and sensory memories are encoded; this is when learning happens.
  • Sleep is a surprisingly complex neurological activity involving synchrony of structures and neurochemicals across the whole central nervous system.
  • Sleep patterns change during our sleep phases and as we mature with age.
  • Rhythmical stimuli of rocking, vibration, stroking, and singing are part of our innate repertoire of human parenting behaviours used to lull babies to sleep across generations and cultures.

What has been helpful to learn more about:

1. Sleep-Wake Synchrony

Our daily sleep-wake cycles are influenced by an interaction of two mechanisms, our homeostatic rhythms, monitored and managed by the hypothalamus, and our light driven circadian rhythms.

Circadian rhythms are timed by our ‘light meter’ the Suprachiasmic Nucleus (SCN) found in the hypothalamus.

Our homeostatic component regulates our sleep need. The longer we are awake and the more physically or emotionally demanding our waking hours have been the greater the build-up of sleep pressure and the more recovery we look for during sleep. The homeostatic load of sleep signalling neurochemicals and hormones can override circadian timing if the need is greater.

2. Sleep architecture

This refers to the basic structural organization of normal sleep. There are two types of sleep, non-rapid eye-movement (NREM)sleep and rapid eye-movement (REM) sleep. NREM sleep is divided into stages 1, 2, and 3, representing a continuum of relative depth. Each stage of sleep has unique characteristics including variations in brain wave patterns, eye movements, and muscle tone.

3. Development of Sleep Architecture

The areas of the brain involved in our sleep-wake circuitry take time to develop, and sleep – wake rhythms may not be fully matured until approx. 4 – 5 years of age.In the womb a baby spends 90 -95 % of the time in a sleep state. Until 32 wks pma sleep is undifferentiated, i.e., no clear states. From term – 3 months babies have just two states of sleep REM and NREM, spending just over 50% of the time in REM sleep. After 3 months NREM sleep matures and 3 NREM states can be noted.For the 1st year infants sleep for more hours than they are awake, and their sleeping hours are chunked into shorter phases (poly phasic) across 24 hours, rather than an adult pattern of extended period at night and this will continue until around 2 + years. Babies sleep need. (For babies born early, sleep requirements are based on their corrected age.)

4. Brainwaves (oscillations) – Spindles and K- complex’s in NREM sleep

Sleep spindles are vital brain waves (oscillations). Their activity is generated primarily in the thalamic reticular nucleus (TRN) of the thalamus. Dysfunctions in the TRN and subsequent spindle formation has been linked to sensory processing difficulties, and sleep disturbance across multiple neurodevelopmental disorders including attention deficit and ASD and is significant in schizophrenia.

Sleep Spindles support 3 critical sleep activities:

  • Inducing sleep by preparing the whole brain for sleep and advancing the brain from N1 to N2 deeper NREM sleep. Reduced density or poorly formed spindles result in delayed onset of sleep.
  • Maintenance of sleep by protecting sleep through ‘spindle gating’. Spindles support the thalamus’ gatekeeping activity by reducing external sensory signals getting to the cortex and interfering with off-line processing during deep sleep. Higher spindle density correlates with longer N2 sleep duration and greater resilience to external sensory stimuli. Reduced density or poorly formed spindles result in frequent awakenings.
  • Supporting memory and learning by sparking increased neuron activity and synapse formations in the hippocampus and cortical areas to enhance memory formation and consolidation and support learning during NREM deep sleep. Reduced spindle activity is a feature of cognitive difficulties.

Spindles are linked to melatonin release. Melatonin starts to be produced around 2 -3 months post term. In the womb it’s the mother’s melatonin the infant responds to entraining circadian rhythm.At birth babies show few detectable sleep spindles until 6 – 8 weeks. By about 3 months sleep spindles last several seconds in duration but are asynchronistic. By 2 years they are synchronised across the hemispheres and two different types of spindle wave become apparent. They reach a peak at about 5 years. Delayed or reduced melatonin production, and delayed circadian rhythm maturation, result in reduced spindles and therefore sleep disturbance. There is current interest in sleep spindles as biomarkers for brain functions and disease risk.

5. K-complex – the largest brain waves

These first occur in infants about five months after birth and continue to develop until adolescence. They are linked to spindles, and like sleep spindles, K complexes are unique to non-REM sleep, and are seen most frequently during the N2 sleep phase. it is proposed that the principal function of K-complexes is the processing of external stimuli during sleep. It is these waves that can be stimulated by sounds, touch and possibly vestibular / vibrations and are responsible for maintaining NREM sleep.The interaction between sleep spindles and K complexes allow the brain to maintain a sleep state, safely ignoring outside stimuli not deemed dangerous whilst making use of rhythmical non-threatening stimuli to support sleep.

6. How does rhythmical stimuli of rocking and vibration help with sleep?

So, rhythmical sensory input part of our innate repertoire of parenting behaviours to support sleep in our young. And it turns out that there is science behind these innate behaviours. It has been demonstrated in several studies that external mechanical, rhythmical sensory stimulation, including rocking, vibration, auditory and tactile input, synchronises with intrinsic brain waves, boosting slow brain wave activity, K-complex and spindle activity, supporting the transition from wakefulness to N2 sleep and maintaining sleep during N2 and into N3 sleep before transitioning to REM sleep.

Rocking (vestibular), some studies have shown that uniform rocking at 0.25 Hz can promote sleep and uniform rocking at 1.0 Hz promotes NREM sleep and reduces REM sleep.

Vestibular stimuli also support parasympathetic expression, leading to improved modulation of arousal state, decreased anxiety, heart rate, and cortisol, promoting sleep.

Vestibular stimulation supports serotonin release both in the SCN and the midbrain dorsal raphe nucleus, the serotonin release then prompts melatonin release by the pineal gland.

Vibration (somatosensory) has been shown to reduce time to move into N2 sleep and increase time in deep sleep.

When we place infants in a cot or pram this sensory feed becomes minimal and if the bed is stationary the vestibular & vibration is not available at all. Interestingly it is quite common for infants to roll their bodies or heads from side to side, even head banging to regulate down to sleep.

Rhythmic auditory stimuli the signature of lullabies, helps moving into sleep by reducing heart rate and breathing patterns.

7. The use of mechanical rhythmical sensory stimuli

Babies are at an early stage of sleep development and science confirms what we’ve innately known as a species that rocking, vibrating, tactile, singing of lullabies can support sleep via the production of sleep spindles/ K-complex brainwaves. By way of a bizarre connection, it seems perhaps that the Brothers Grimm already knew this, their Sleeping Beauty slipping into a deep and extended 100-year sleep with the aid of a spindle!It seems that mattress-based mechanical rhythmical sensory stimulation does provide a means of synchronising brain waves, facilitating K-complex and increasing fast sleep spindles, thereby supporting transition into deep sleep so benefiting the baby and the exhausted parent. It should be noted that most studies are on adults or older children/ young people. Note however that vibrating mattresses are being looked at for use in neonatal units to support sleep. The studies we reviewed did not address whether, when the stimulus is later removed, sleep became more difficult or there was impact on future sensory development.

And so back to the question. What might the impact these sensations have on the child’s sensory development?

The answer from is we really don’t know. Sleep is essential and like learning to walk and talk it takes time to for our neurophysiology to mature and for us to entrain the skill. Understanding this may help reduce the social/ psychological pressures around sleep. Is mechanical facilitation of the required rhythmical sensations for sleep a better bet than having a ‘village’ around the new family so that an adult has the physical and emotional capacity to support the poly-phasic sleep of their baby? This as always depends on individual circumstances. For many, parenting alone, especially during the day, is a reality. But there is more work needed to establish whether there is research that informs an answer to the benefits or otherwise of extended exposure to mechanised sleep-facilitating equipment.

8. Postscript: Questions to Ponder

  • Babies and young children have a much shorter sleep cycle than adults:
  • Newborns have a sleep cycle that lasts approximately 40 minutes.
  • At a year this increases to 50 minutes.
  • Toddlers and very young children have a sleep cycle that lasts approximately 60 minutes.

These cycles include NREM and REM sleep. Both are important. It is not clear how the presence of continuous rhythmical sensations impact REM sleep and if consistent mechanical input impacts ability to move into REM sleep.

Consideration about strength of vibration, length of time of the stimulus and the interaction of these factors would seem important. We would highlight here that when swaddled, babies should be able to free their arms. Babies should not be left to sleep in baby seats or car seats, should follow the ‘Back to Sleep’ advice and babies should sleep in their parent’s room for at least the first 6 months. We refer parents to the safer sleep advice from the Lullaby Trust.

Few studies have looked the sensory environment of the parent to support sleep. One study looked at parents holding a crying baby to get them down to sleep. Critical was walking around for 5 – 8 minutes and then sitting and holding for 5 minutes before putting the baby down. Holding and walking and then sitting still and holding were both important components. We know that for premature infants, access to skin-to-skin promotes sleep.

When we rock our babies in our arms or in carriers, we have the possibility of providing multi-model synchronised sensations of vestibular, somatosensory, smell, auditory and the rhythm and intensity can be attuned to need by observing behavioural cues. By interacting in this way co-regulating neurochemicals of oxytocin can be produced which support stress reduction and attachment. When parents are highly stressed however co-regulation activities become highly charged creating dysregulation rather than the calm required by all to support slipping into the land of nod.

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