Polynomial $ P(t)$ is such that for all real $ x$,
\[ P(\sin x) \plus{} P(\cos x) \equal{} 1.
\]
What can be the degree of this polynomial?
1. Given the polynomial \( P(t) \) such that for all real \( x \),
   \[
   P(\sin x) + P(\cos x) = 1
   \]
   we need to determine the possible degree of \( P(t) \).

2. First, observe that \( P(-y) = P(\sin(-x)) \). Since \( \sin(-x) = -\sin(x) \) and \( \cos(-x) = \cos(x) \), we have:
   \[
   P(-y) = P(\sin(-x)) = 1 - P(\cos(-x)) = 1 - P(\cos x) = P(y)
   \]
   for all \( y = \sin x \in [-1, 1] \). This implies that \( P(t) = P(-t) \), meaning \( P(t) \) is an even function.

3. Since \( P(t) \) is even, it can be expressed as \( P(t) = Q(t^2) \) for some polynomial \( Q \).

4. Substituting \( P(t) = Q(t^2) \) into the original equation, we get:
   \[
   1 = P(\sin x) + P(\cos x) = Q(\sin^2 x) + Q(\cos^2 x)
   \]
   Using the Pythagorean identity \( \sin^2 x + \cos^2 x = 1 \), let \( y = \sin^2 x - \frac{1}{2} \). Then:
   \[
   Q\left(\frac{1}{2} + y\right) + Q\left(\frac{1}{2} - y\right) = 1
   \]
   for all \( y \in \left[-\frac{1}{2}, \frac{1}{2}\right] \).

5. The polynomial \( Q\left(\frac{1}{2} + t\right) + Q\left(\frac{1}{2} - t\right) - 1 \) has infinitely many roots, implying it is identically zero. Therefore:
   \[
   Q\left(\frac{1}{2} + t\right) + Q\left(\frac{1}{2} - t\right) = 1
   \]

6. This implies that \( Q\left(\frac{1}{2} + t\right) - \frac{1}{2} = -\left(Q\left(\frac{1}{2} - t\right) - \frac{1}{2}\right) \), meaning \( Q\left(\frac{1}{2} + t\right) - \frac{1}{2} \) is an odd function. Therefore, we can write:
   \[
   Q\left(\frac{1}{2} + t\right) - \frac{1}{2} = tR(t^2)
   \]
   for some polynomial \( R \).

7. Substituting back, we get:
   \[
   Q(t) = \left(t - \frac{1}{2}\right) R\left(\left(t - \frac{1}{2}\right)^2\right) + \frac{1}{2}
   \]
   and thus:
   \[
   P(t) = Q(t^2) = \left(t^2 - \frac{1}{2}\right) R\left(\left(t^2 - \frac{1}{2}\right)^2\right) + \frac{1}{2}
   \]

8. The degree of \( P(t) \) is determined by the degree of \( R \). If \( \deg R = n \), then:
   \[
   \deg P = 2 + 4n
   \]
   Therefore, the degree of \( P(t) \) can be any natural number of the form \( 2 + 4n \) for \( n \geq 0 \).

9. Additionally, if \( R \equiv 0 \), then \( P(t) = \frac{1}{2} \), which is a polynomial of degree 0.

Conclusion:
The degree of the polynomial \( P(t) \) can be 0 or any natural number of the form \( 2 + 4n \) for \( n \geq 0 \).

The final answer is \( \boxed{ 0 } \) or \( 2 + 4n \) for \( n \geq 0 \).