Why the supply curve of Labour in backword sloping? Explain 

In economics, a backward-bending supply curve of labour is a graphical device showing a situation in which as real (inflation-corrected) wages increase beyond a certain level, people will substitute leisure (non-paid time) for paid worktime and so higher wages lead to a decrease in the labour supply and so less labour-time being offered for sale. 

The "labour-leisure" tradeoff is the tradeoff faced by wage-earning human beings between the amounts of time spent engaged in wage-paying work and satisfaction-generating unpaid time such as sleep.

Such a comparison generally means that a higher wage entices people to spend more time working for pay; the substitution effect implies a positively sloped labour supply curve. However, the backward-bending labour supply curve occurs when an even higher wage actually entices people to work less and consume more leisure or unpaid time. 

Higher pay for overtime hours can reduce or negate the effect of a backward bending labour supply curve, by increasing wages only for hours worked beyond a certain amount. Overtime maintains the substitution effect at a high labour supply. However, the income effect from the wages increasing on all the previous hours worked is eliminated. Thus, higher hourly overtime pay can cause workers to work more hours than if the higher rate is paid on all hours.