When do words need a capital letter in the middle of a sentence?
If it is a proper noun.
Proper nouns are a special type of noun (naming word). It describes something that is unique. It can be: places, people, institutions, companies and brands.

[1] Places
South Pole, North Pole, Antarctica, Africa, America, the Americas, France, London, Little Tooting, Springfield, the Meridian line, the Equator, North East England, the North East, the South, the Baltic Sea, the North Sea, the Thames, the Amazon, the Nile, the Tropic of Cancer, the North Sea, the Baltic, the Urals, the Pyrennees
NO Need for Capitals: home, the world, worldwide, the seas, the ocean, northern hemisphere, the tropics, equatorial regions

NO Need for Capitals school, university YES the University of Sheffield, the School of Science, Chiswick Secondary School

NO Need for Capitals go west, travel south, due east YES the East, the West

NO Need for Capitals the environment, the sky, the sun, the moon, space, the planet YES Sol, Luna, Jupiter, Saturn, Sol, the Earth, Neptune, the Universe

[2] People
Bob Smith, Georges Luis Borges, Vladimir Nabokov, Yuri Gargarin, Nelson Mandela, Chinua Achebe, the President, the Prime Minister, the Chancellor of the Exchequer, the CEO (Chief Executive Officer), the Head of Department, the Major, the Head Chef, the Head Surgeon, the Headmaster, the British, the French, Africans, Americans
NO Need for Capitals: the accountant, the lawyer, the policeman, the boss, the typist, the nurse, the doctor, headmasters, police, southerners, northerners

[3] Time

Monday, Friday, Sunday, January, June, November
NO Need for Capitals tomorrow, today, one o’clock, eleven twenty, noon, midday, midnight

NO Need for Capitals spring, summer, winter, autumn YES Autumn/Winter 13 Fashion Collections

[4] Institutions, Companies, Products

the University of Oxford, the Faculty of Biological Sciences, the Government, the Legal System, Apple, Macbook, Microsoft, Intel

NO Need for Capitals governments YES the Government, the French Government

NO Need for Capitals the police YES Greater Manchester Police

NO Need for Capitals hoover, vacuum cleaner YES Dyson

NO Need for Capitals cola YES Coke

Sometimes part of a phrase has capitals and part doesn’t:

 south-east London
English history
British novels, African poets, American film makers
Amazonian rainforest
the London sewage system
Britain’s rivers