Is it True that People Born in Summer Need Less Sleep than People Born in Winter?

Why do some people seem to need less sleep than others? Why is it so easy for some people to get up in the morning and for others it's easier to stay up late at night?

As far as we know, there is no data on the association between sleep patterns and birth dates. I even asked an expert on the topic: Prof. Dr. Till Rönneberg from LMU Munich. However, he told me that this has not been investigated in depth. What has been thoroughly investigated though is why some people are early birds and some are night owls. This has to do with our inner clock that has evolved to help us anticipate the daily changes in our environment, such as light-dark cycles or temperature cycles. Almost everyone’s inner clock is happily ticking away with an approximately 24-hour period (that is why it is termed “circadian clock”- circa dias - almost a day). However, the ticking speed varies. Imagine you have a fast inner clock. This means that you would be too early for everything: you wake up early, eat early and get tired early; you would be an early bird, or as the scientific community terms it - an early chronotype. The opposite is also true; late chronotypes have a slow inner clock, so they are “late” for everything.
The interesting thing is that our chronotype varies throughout our life; we all start off as very early chronotypes, our clocks become gradually slower until puberty and then getting earlier again during the rest of our lives. Not only is our chronotype age-dependent, but also sex-dependent.
These differences in individual chronotypes are definitely something which we have to keep in mind when discussing school hours or office hours. These are often much too early especially for school children in the midst of puberty.
Last but not least, I want to mention an important feature of our inner clock, and that is that it is able to synchronize to our environment through light. This means that it can entrain the 24-hour day-night cycle provided it receives enough light input. To cut it short, the more light we get, the easier it is for night owls to get up in the morning and the longer early birds can stay up at night.

Reference

Rönneberg and Merrow, Cold Spring Harb Symp Quant Biol, 2007

by Veronika Lang
this article originally appeared 2013 in CNS Volume 6, Issue 4, Integrative Medicine

Do you also sometimes wonder about the simple neuroscientific questions in everyday life, but don’t really feel like looking them up right away? For questions like this, just mail us your question (cns-newsletter@charite.de) and Dr. Harebrained will give us his explanation in the next issue!

Why is it again that we yawn?

This question is probably as old as yawning itself and still remains to be answered, although there are a lot of interesting hypotheses out there:

1. Respiratory and circulatory hypothesis
The most widely believed and oldest hypothesis that yawning functions to regulate levels of oxygen and carbon dioxide. However, studies by Provine and colleagues showed that when measured in a controlled environment, yawning frequency was not affected by manipulating levels of oxygen and carbon dioxide [1].

2. Mental attribution hypothesis and social/communication hypothesis
This hypothesis claims that just thinking about yawning makes you yawn [2] and is closely linked to the social/communication hypothesis. Yawning is understood across cultures and frequently occurs in social contexts. Moreover, everyone has experienced a yawn being "contagious". Interestingly, the susceptibility to contagious yawning correlates with empathic skills. This is reduced, for example, in patients with autism or schizophrenia. However, experimental data are lacking and highly debated [3,4].

3. Arousal hypothesis
This hypothesis proposes that yawning facilitates arousal. The experimental data suggest that yawning indeed occurs during progressive drowsiness or sleepiness, which would be compatible with the hypothesis. However, EEG data could not show specific arousing effects of yawning on the brain or the autonomic nervous system [4].

Macaca fuscata juvenile yawning via Wikimedia Commons

4. Thermoregulation hypothesis
This is one of the most recent hypotheses and puts forward that yawning functions as a brain cooling mechanism. The mechanism proposed being an increase in brain temperature triggers a yawn and the physiological reactions following promote a return to brain thermal homeostasis. However, again, experimental data remain sparse and highly debated [3,4].
Nevertheless, here are a few facts that we know for sure: Yawning is a phylogenically old property and occurs in four vertebrates classes: mammals, reptiles, birds and fish. A yawn is characterized by a standard cascade of movements over a 5- to 10-second period. The mouth opens widely for 4 to 6 seconds with simultaneous retraction of the tongue, and is usually combined with retroflexion of the head and sometimes elevation of the arms. Numerous neurotransmitters, neuropeptides, and hormones have been found to be implicated in the control of yawning; amongst others, dopamine, acetylcholine, glutamate, serotonin, nitric oxide, adrenocorticotropic hormone related peptides, oxytocin, and steroid hormones facilitate yawning, whereas opioid peptides have an inhibitory effect [4].

What is there to be concluded? 
Ladies and gentlemen, there's research to be done!
Do you also sometimes wonder about the simple neuroscientific questions in everyday life, but don't really feel like looking them up right away?

Do you also sometimes wonder about the simple neuroscientific questions in everyday life, but don't really feel like looking them up right away? For questions like this, just mail us your question (cns-newsletter@charite.de) and Dr. Harebrained will give us his explanation!


[1] Provine et al., Behav Neural Biol, 1987
[2] Provine, Ethology, 1986
[3] Gallup, Neurosci Biobehav Rev, 2011
[4] Guggisberg et al., Neurosci Biobehav Rev, 2010


By Veronika Lang, originally published June 2012 – Volume 05, Issue 2, "Open Access"

Why is light reflected off animal eyes at night, but not humans?

While on my way home yesterday, two eyes were staring at me out of the bushes. What is the reason for this scary phenomenon?


The phenomenon of 'eye-shine' is seen in a variety of species, and is thought to be due to the presence of an intraocular reflecting structure, the tapetum lucidum. This structure was described as early as 1854 in the Medical Lexicon by Robley Dunglison:

"TAPETUM, Ta'petry, Tape'tum choroidea. A shiny spot, on the outside of the optic nerve, in the eyes of certain animals, which is owing to the absence of the pigmentum nigrum, occasioning the reflection of a portion of the light from the membrana Ruyschiana."

Raccoons in a tree with their tapeta lucida reflecting camera flash

The Tapetum Lucidum
And he wasn't too far off back then. The tapetum lucidum is basically a biological reflector system that is a common feature in the eyes of vertebrates. Some species, usually diurnal animals (primates, squirrels, pigs), do not have this structure. Depending on the species, location and composition may differ; the tapetum lucidum can either be located within the retina or within the choriod behind the retina. The reflecting material in the tapetal cells also varies from guanine crystals (in some teleosts, reptiles) or lipid (in some teleosts, some mammals) to zinc cysteine/Riboflavin/collagen (most mammals). Therefore, not all animals eyes glow the same color, ranging from yellow, to green, to blue, and everything in between. Age and light angle can also affect the color of the eye shine. The tapetum functions like a mirror and provides the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation, thereby enhancing visual sensitivity at low light levels.

The Red Eye Effect
But wait a second... what about the 'human' red eye effect in pictures? Interestingly this effect rarely occurs in animals that have a tapetum lucidum; however, as we all know, it often does in humans. The cause of it is that the light flash occurs too fast for the pupil to close, allowing much of the very bright light from the flash to pass through the pupil into the eye. It then reflects off the fundus at the back of the eyeball and out through the pupil. The red to orange color which we then observe on photographs is due to the reflection of light from choroidal blood vessels.


Do you also sometimes wonder about the simple neuroscientific questions in everyday life, but don't really feel like looking them up right away? For questions like this, just mail us your question (cns-newsletter@charite.de) and Dr. Harebrained will give us his explanation!

Dunglison, Medical lexicon, p. 843., 1854
Ollivier, Vet Ophthalmol, 2004

By Veronika Lang, originally published March 2012 in Volume 5, Issue1, "Mental Health Disorders"