While  children  should  form  habits  of  industry,  they  should
not  be  subjected  to  strain  of  any  kind.  To  the  extent
that  bodily  welfare  permits,  the  young  person  is  to  be
trained  mentally,  but  education  must  not  be  at  the  expense


456  PRINCIPLES   OF  HEALTH   CONTROL

of  bodily    development.     Variations    which    need   to  be
observed  on  account  of  age  include  the  following:

1.  As  to  sleep.  --The  infant  should  sleep  most  of  the
time,  but  from  this  period  to  maturity  the   amount   of
sleep   should  be  gradually   diminished.     Seven   to   eight
hours  a  day  are  required  by  the  average  adult,  but  chil-
dren under  twelve  require  at  least  ten  hours.     Old  people
require  perhaps  less  sleep  than  those  in  middle  age,  but
never  more.
2.  As  to  exercise.  —  In  no  period  of  life  should  the
amount   of  physical   exercise   exceed   one's   capacity  for
generating    energy.     Because    the    old    person    has    less
energy  for  external  use  than  the  youth  or  the  one  in
middle  age,  he  must  take  less  exercise.      On  account  of
degenerative  changes  in  his  tissues,  he-  must  also  omit
exercises   of   a   violent  nature.     The   old  person   should
not  make  the  mistake,  however,  of  curtailing  exercise  to
the  point  of  withdrawing  a  needed  stimulus  to  his  vital
processes  (page  221).
3.  ^4 s  to   mental  work.  —  Mental   work    of    a    serious
nature  may  be  begun  at  the  age  of  eighteen  or  twenty
and   continued  throughout  life,  provided  the   conditions
necessary    to    all-round    bodily    vigor    are    maintained
(Chapter  XV).
