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After reading the discussion of the traditionally accepted 8 hours of sleep a night versus a possible 6.5 hours as optimal for maximum brain volume, I wonder if the difference may be linked to our measurements. Before we had smart devices with sleep trackers, we would generally assume that the time we went to bed …

John Kitchen pushes the idea that Victorian homes (broadly) aren't suitable for heat pumps, suggesting they may have to be bulldozed. Heat pumps simply output heat, and physically can heat any building, full stop. It is true that cost, unsightliness and regulations may be obstacles, but these are artificial constraints. Equally, all buildings are far …

I appreciate the more realistic approach to so-called ultra-processed foods in your recent article, but would like to make a couple of comments ( 28 October, p 40 ). Emulsifiers aren't all potentially harmful "additives" as implied. Many occur naturally in food we eat, such as lecithin (E322), found in egg and soya beans, and …

Ever since humans walked out of Africa, migration has taken place. However, when the land is already occupied – and the stronger group makes no attempt to treat the other with compassion, tolerance and respect – long-term problems arise. The Indigenous peoples of North and South America, Australia and New Zealand, among others, bear witness …

On the subject of acne, rather than lotions or medicines, I find the easiest way to improve skin is to exercise for about 30 minutes non-stop and work up a good sweat. Then sit in a steam room for as long as you can with a bottle of water for rehydration. My skin improves in …

You report that ancient Europe before any real human influence was full of savannah, grazed by elephants, along with a suggestion that if conservationists want to restore the ancient forest biome, we should recreate the processes that led to this landscape. However, is this Eemian interglacial landscape an appropriate or realistic biome model for conservation? …

In your review of Ben Goldfarb's book on wildlife-traffic conflicts, the figure of 2 billion new road vehicles by 2030 was depressing. Sadly, the mindset seems to be business as usual with the excuse that it will be "green". Everyone having their own car and the road capacity to use it is unsustainable, no matter …

The rooster that wasn't alarmed by its own reflection in a mirror may not be exhibiting self-awareness so much as "not-other awareness". Any species that drinks from still water will be presented with a natural mirror, and it will need to know that the reflection it sees isn't a foe to be fled from every …

The progress in South Australia on renewable energy is fantastic. But it would be useful to get away from lithium-based batteries for grid storage. This would decrease the demand and hence the price of this metal, reducing further the cost of electric cars to consumers ( 28 October, p 36 ).

Graham Lawton writes that "coal ash alone contains an estimated 288,000 tonnes of lithium, enough to supply the US market for 130 years". But just switching UK cars to lithium batteries would require 50,000 tonnes of lithium, and switching the whole world would require 40 times as much ( 18 November, p 22 ).