The necessary mental pre-condition of sleep is the concentration of the ego upon the wish to sleep and the withdrawal of psychical energy from all the interests of life; since at the same time all the paths of approach to motility are blocked, the ego is also able to reduce the expenditure of energy by which at other times it maintains the repressions. 
- Those which have not been completed during the day, owing to some accidental cause.
- Those which have been left uncompleted because our mental powers have failed us, i.e. unsolved problems.
- Those which have been turned back and suppressed during the day. This is reinforced by a powerful fourth group:
- Those which have been excited in our Ues. during the day by the workings of the Pes; and finally we may add a fifth, consisting of:
- The indifferent impressions of the day, which have therefore been left unsettled.